The reason because any calculation, based on the splits of the race, can give some idea, but only regarding a range, not very wide but not too much narrow, is that, for different athletes, we can find a different kind of physiological attitude.
For example, if we look at top specialists of 800m, we can see the athletes able to have the best kick when the race is slow are not the fastest, coming from 400m, but the "aerobic", having more aerobic capacity. In other words, the final speed doesn't depend on the absolute speed of the athlete, but on the level of tireness he has at that point of the race.
The ability in kicking is something connected with morphology af the athlete, his running technique (depending on his morphology, his neuromuscle ability, his reactive ability, his tactical ability) and his aerobic level. The ability in running very fast at even pace is due, mainly, to the level of specific endurance the athletes has for his event.
An athlete very tall (like Juantorena, Johnny Gray or Fiasconaro, for instance) can run fast using a very high pace. This kind of runners can run 400m in 45" or less, but don't have ability in changing quickly their speed, because don't have a high frequency in their action. When you have to change speed, the first technical action to do is to "reduce the gear", exactly the same we do with a car, whenm if we want to accelerate in short time, we need to enhance the number of revolution, and we move to an inferior gear. That's the reason because, in the history of top championships, athletes not very tall, and sometimes not having top performances, were able to win in tactical competitions.
An athlete coming from 400m at that level has a very strong lactic ability (can overtake a level of lactate in his muscles of 20-22 mml), fruit of a high LACTIC POWER, but doesn't have a good ability in removing quickly his lactate, and has a very low anaerobic threshold. Fiasconaro, able to beat the WR in 1973, completely alone, with 1'43"7 (24"8 / 50"1 / 1'16"5, so 24"8 / 25"3 / 26"4 / 27"2), had a AnT at that time of 16,8 km/h. This means that, at a pace of 3'34" per km (5'44" per mile) he already was in a lactic area. His attitude was the ability in producing very much lactate in short time, and to be able to have a high level of tolerance of lactate in his fibres, typical of a strong lactic engine. For winning a race, these athletes must run fast from the beginning, because their goal is to "kill" before the end the aerobic runners, forcing them to reach very quickly a high level of lactate, that doesn't allow them in changing speed.
In this type of runners, every calculation based on the control of their splits cant give real results, because they are competitive for running fast from the beginning, but at even pace.
Of course, different evaluation can be done considering longer distances, because the typology of top specialists of 10000m, for example, is more similar.
In any case, it's true that the most part of top athletes was not able to reach their potential pick during the career. Many times their pick was in occasion of top championships, but at the end the final goal was to win, not looking for a fast time. Other times, there was some problem during the attempt of record.
Personally I have the experience of Saaeed Shaheen. In 2006, his shape was very much better than in 2004, when he beated the WR in Bruxelles. During that meeting (he had a top motivation, because the Kenyan Olympic Committee didn't give him the release for representing Qatar in Athens 2004, so we decided to try the WR immediately after Olympic Games) he finished in 7'53"63 with last 100m very relaxed, because he could see the big watch 5 meters after the finish line, and could control the fact that the record was already sure. That day, he could run under 7'53" without any doubt.
In 2006, we had a period of incredible training. His shape was 20% better than in 2004 : he was faster, more resistant at high speed and in long distance, and better in hurdles technique. When we speak about a WR, never we can be sure about the possibility to better it, but that time I was practically sure that beating the WR was the slightest goal to reach : the real goal was to try to run 7'50".
What did happen ? In 2004 Shaheen had perfect pacers till 1800m (even pace, very good technique) and the weather was very nice. In 2006 there were not good pacers : before the bests were Le Dauphine, Vroemen, Luis Miguel Martin. In 2006 these athletes were no more able running fast. We had in Zurich 3 rabbits : the first till 1200m, the second till 1600m and the third till 2 km. About the third, I spoke with Mateelong (also another my athletes) asking him to renounce his race for helping Shaheen, and I supposed this was the best solution (Mateelong was the number 3 in the World at the moment). I told Shaheen "with Mateelong third pacer, don't try anything before the second km, also if the pace slows down a little". But I never could suppose what happened. The weather was not good (very cold and windy). The first 4 laps were ok (about 62"8 each one, perfect pace for running 7'52"). Mateelong went in front after 1500m about, but was not able to maintain the same speed, slowing down with one lap in 68"5 ! So, at the end, Shaheen lost motivation, and finished in 7'56"64 only, not pushing in the last lap for saving energies for another attempt, next week in Bruxelles.
In Bruxelles, 30' before the race the two pacers (from Spain) came to me telling me that they didn't feel able to run fast as we needed, so he was completely alone from the beginning, and only went to win.
So, I really know he could run 7'51", because I have all the parameters in his training before, during and after his WR. But there is no analysis of any split that can support the same sensation. That, in any case, remains a sensation, because in the book of records is still written 7'53"63.