Designers call this a "false bottom" and try to make sure it doesn't happen, but it can be a near impossible task to make sure there's never one given variation in screen sizes / orientations.

Often you see websites with hero areas and panels that fill the full screen on standard desktop sizes, but attentive teams will make sure something is cut off so that the page looks unfinished and people scroll.

For what it's worth, I've also seen data that Apple users are more likely to scroll by default simply because they've made the scrolling experience so pleasant.

Or maybe they're scrolling by default because they're used to not knowing whether or not content is scrollable?

Yup, that's possible too. Probably a bit of both TBH.

Though again, from user testing sessions I've observed, I'd say that "not knowing whether or not content is scrollable" can be a universal problem, not limited just to Mac users.

Interesting. Would you suggest to have the scrollbar visible?