I asked myself same question time ago (2017 iMac, 580Pro).
It took me some time in the web to find that "Pro" in this case is not pro. These are special cards made for Apple (perhaps others too) and they aren't AMD Pro line of v-cards.
Mine is just a Radeon 580x working with slightly less voltage, minor frequency, less heat. Why is named "Pro"? Who knows.
Apple does that for two reasons: One is to increase long term reliability. Th other is so that wider tolerances allow them to use silicon that tests on the lower end of the spec—until tested, no one knows the exact parameters of an individual chip. By allowing a wider acceptable spec, they get a lower overall price. Apple does this with Intel CPUs, too—that's why the clock speeds are lower than those same CPUs on the shelf at Fry's.