Is this on a server?
Some servers run an addon that combines dropped entities together for performance reasons. So if you drop 30 torches on the ground near each other, they'll combine into a single stack of 30 torches. I suspect this is what's happening with your armor (not sure why, since combining armor is a recipe. But that's my best guess)
Sorry, yes this is on a multiplayer server. I understand the grouping of drops into stacks but I didn't think this worked to combine broken armor into fully repaired armor. It also happens with enchanted armor. I get only broken pieces if the chest is empty but if I cleanup after I get fully repaired pieces of enchanted and unenchanted gold, leather and chain armor.