I studied this back in the day. When looking at published size restrictions and the then bin sizer they used to use frequently at airports to "test" your bag size - there was an additional 1/2-1 inch extra room.

I have both a Travelpro international carry on (1-2 days for me) and a Domestic Carry on (2-3 days for me). I have found both fit in the over heads of most aircraft which is the main thing anyway.

Now lets talk about personal item backpacks and THOSE pesky under-seat restrictions. Just kidding - no one talks about those sizes here.