What's going on is that practicing for programming competitions (which is needed to perform well in that venue) gives you a leg up on Google-style interviewing

What's going on is that practicing for programming competitions (which is needed to perform well in that venue) gives you a leg up on Google-style interviewing skills, but not a leg up on Google-style day-to-day demands. As a consequence, among those who do get the job, those who were good at competitions have a correlation to not being quite as good at the day-to-day, since there are some in that category who would not have got the job without the competition experience.