>Physics degrees still mean you have very decent maths, but CS degrees are not necessarily as strong a signifier of it.
That and they necessarily have to learn a lot of CS just to do their own degree work. It's like wondering why cross country runners perform well at other sports too, their sport incorporates the 'hard' part of most sports, the running and endurance aspects.
Some CS programs have moved away from heavy maths requirements over the last 2 decades, outside a few maths-heavy specific CS courses.
Physics degrees still mean you have very decent maths, but CS degrees are not necessarily as strong a signifier of it.
I suspect that is the difference, particularly if you are actually working on tricky programming tasks.