When I read about the tire sizes people use it´s pretty common to see the same profile height used on both 17" and 18" wheels. That way the 18" tire is bigger in total than the 17", sometimes even OEM profile combined with wider tire AND bigger rim.
Is there a reason for this? Bad roads? Comfort? OD too small from factory? Hard to find "correct" sizes?

On my old cars I´ve always matched the total tire height (or OD) to the OEM size to keep the speedometer showing correct numbers and keep the gear ratio correct.

Not trying to piss people off now, just a question since I sell tires and want to learn the best option for each car.