I think many people are faced with a choice of studying physics or chemistry at a graduate level and, since up until then chemistry is generally much harder work than physics if you are an above average student, gravitate towards the subject where their ability so far has made life easy for them.

I still remember my introductory chemistry lecture at uni where the instructor said that a whole bunch of us had coasted up until now, and that in his experience those that had had to work hardest to get there, even if they had lower grades, would be the ones to succeed (he was right!)