It's a bug. The current description is that they're hoping to reclassify it as a "feature".¨ "Clearly" the fact that they introduced another item, the 10-stack of heavy ammo, didn't mesh well with the current code that said "oh, we have 5 slots for items here". This is just bungie being bungie. Nothing is a bug until it is documented, and when it is documented is is now a feature, clearly we don't have bugs in Destiny.