In my experience too, "reloadable" means the mag can be used to do an active reload while moving through the stage. You might be able to squeeze another round into it, but if you try to insert it in the gun there isn't enough room for the stack of rounds to move downward when the top round presses against the underside of the slide, and it won't go in. It may fall right out or not, but it won't latch and won't feed any rounds. These mags are sometimes termed "20-round capacity, 19 rounds reloadable".
This doesn't just happen to 1911s or STIs (2011s). Glock has been playing with their mag design, and it's getting harder to have a full reloadable capacity. I have some new .40 15-rd mags that you can push 15 into, but they don't reload reliably, and several new .40 10-rd mags that definitely won't reload, even after letting them sit fully loaded for a month. There are several threads here on BE on how to "adjust" the mags, but the bottom line seems to be that they're coming from the factory a lot tighter than they used to.
I once screwed myself totally in a AR-15 stage by using borrowed mags, loading them right to the brim, and then couldn't do a reload at all until I took one round off the top. It simply wouldn't latch into the gun. Embarrassing, but a valuable lesson. If loading to "capacity" I always push down on the top round to see if there's space to compress when it goes into the gun. If I can't compress it, one round comes off.
HTH