The game is rigged to give you ammo types you don't have. It's always done this. The ammo dropped is determined by who killed the enemy. so when everyone is shooting you'll see lots of useless primary and secondary ammo because the person who got the kill shot was either low on that, had a weapon equipped that causes more of that type to drop or has run out of that type of ammo. Of course if you think it's a glitch then Bungie can always patch it so that far less heavy drops for everyone and then you'll need to be Xur's best friend or have a lot more glimmer farming in mind.