You just are not allowed to compete with more than 15 in a mag in P and PO. If you do, you're shooting in open division instead.

Remember standard and open guns are unlimited in capacity but must fit in the box, making the magazine length, baseplate, follower and spring (and caliber) a contributor to theoretical performance. Fewer mag changes = better score all else equal

So in P and PO, they take production guns with a range of capacities and limit that capacity to an amount lower than average so more people can compete with what they have got and its less of an equipment race in theory