People lose height as they get older. Foreman probably lost an inch or two in height by the time of that picture.
Most heavyweights seem to upgrade by an inch. Fury is a shade under 6'8", Wilder just under 6'7" AJ is like 6'5.5" maybe they just get their height measured in their trainers or something. It's worse in the lower divisions I've seen a guy claiming 5'6" fight a guy claiming 5'11" and they were the same height.
He was in his prime. My dad is now 70 years old and he has lost around 2 inches of height compared to his prime.