Well, I believe that you believe that you can know that. However, there is not only one alternative. Also, physiological variables actually change from one marathon build-up to the next. At least they should. Accumulated aerobic development, to name a big one, unless it's really not that significant in this case. That alone could certainly allow what previously had been "blowing me" (interesting phrasing, even for this site) pace to be perfectly reasonable in a subsequent race.
Or do you wait to see what the next mile and/or 5km marker indicates instead of getting in a panic over a single mile split? It's splitting hairs, mere rationalization. The subsequent mile or 5km marker would have probably yielded the same insight, as well. Unless it was a total crap race with really randomly misplaced markers all over the place in which case anyone who relies on mile markers for their racing didn't do their homework.
Strawman. I said nothing about changing routine on race day, specifically. In fact, the term "routine" should have indicated otherwise.
So, even with firm knowledge of that fact, you didn't trust your preparation?
Granted and I did note that the first maybe 5 miles of that course drop very little compared with the middle miles. I was talking about overall. Look at about any race report people post online, they routinely contain splits for every mile as if there's any subtle clue to be found within the hallowed data. If you fall apart, especially in a marathon, you usually know it without even needing to look at a watch. Same deal if you have too much left at the end.
Then they are wrong and need to address that fallacy and shortcoming in their training.
Wow, I hope this defeatist attitude doesn't rub off on too many others reading this. I hope people take cues from people like Bill Rodgers and Steve Jones rather than Joe Downer the cliff-diver.