Exactly. It's simpe thermodynamics: spend more energy than you consume, you lose weight. Consume more energy than you spend, you gain. Now what unique blend of will-power, feeling satiated, availability of good food, and amount of time people are willing to spend exercising is highly variable.
Take 2 people: 1 has a job where taking 1.5 hours for a lunch-time run is not an issue, lives in an area conducive to exercise (trails, good weather, etc), has money to spend on good, in-season food, and can easily bike/walk to work or run errands.
The other has a job with a set 30 minute lunch, no shower or locker-room there, lives in a crappy area for running, makes not much money so buying whatever fruit/vegetable looks good is not always an option, and has to drive a long enough distance to work/errands, such that walking or biking is not practical for time reasons.
Both are governed by the same laws of thermodynamics, but the first person will have it a hell of a lot easier adjusting their life to keep from gaining weight. I'd love to see more places have good facilities to accomodate bike (or running- a coworker runs to/from work most days) commuters, be flexible with work-hours, and basically make it easier for people to make the healthy choice. I routinely bike to work, and at least have a locker-room with 2 showers, but if the place was serious about helping employees stay active there would be more showers (there's often a wait), more lockers (I can't get one, so have to plan ahead a little for changes of clothes), and they'd stop locking the door by the bike racks early...