"Why do gyms allow other people to put Pokemon in them while under attack, but not me?"

#PokemonGO: There's a gym that I can reach while in my apartment, and as such I battle a lot at that gym. However, whenever there aren't 6 Pokemon in the gym and I try to put one in, it usually says I can't because the gym is under attack. BUT whenever I attack the gym and kick out one or more of the Pokemon in there, while attacking it there always seems to be another Pokemon that magically appears that wasn't there when I started attacking the gym! Why is this??? I thought you can't place Pokemon in gyms if they're under attack? Or at least I can't....Also when I attack a gym alone, I get so close to taking it, only for it to fully fill with Pokemon and all the Pokemon's CPs are back to the highest level..... It's so annoying and I usually just give up at that point, because there are obviously people that care about keeping their Pokemon in it than I do. If I do take it, it's taken down within minutes anyways. It's not really worth the effort to be honest... via /r/pokemongo https://ift.tt/2HFt4dN