If some mags are lucky enough to hold 13 rounds, it isn't because they have orange followers. The only difference between the black 69 followers and the orange 59 followers is color.

I have a few 59 mags w/ black followers that hold 15 rounds but they cannot be locked in to the magwell with the slide closed, too full.

Original 59's held 14 rounds, as did 459/659's. When they introduced the mini-gun, I suspect they had to modify the follower so it would hold 12 rounds. Later someone probably realized that putting the 69 follower in a 59 mag increased the capacity to 15 rounds so they started making them in orange to show the difference. That's my theory and I'm stickin to it.

I have 14 round 5904 magazines that have 69 followwers in them so they hold 15 rounds but maintain the correct look of the black followers.