There are several of my nice new breastplates scattered around, and I'm not sure how they got to where they are.
Let's suppose you have one military squad with one dwarf in it. You assign a uniform to this squad/dwarf, and that uniform includes one helm, one breastplate, etc.
Now you tell the armorsmith to make a breastplate. The job gets finished, and then the breastplate, while it's still sitting in the forge, either gets marked for a "Store Item in Bin", "Store Item in Stockpile" or "Pickup Equipment" job, depending on which dwarf gets wind of it first. Or something. If the military dwarf wins, he does the Pickup Equipment job, and puts on the breastplate. Otherwise, it goes into a stockpile and then eventually the military dwarf grabs it from there.
Then later you make a second breastplate. This one's better than the first one, so the military dwarf decides to claim it (assuming a hauler doesn't grab it first). But the military dwarf is already wearing a breastplate -- that would prevent him from performing the Pickup Equipment job! So he drops the breastplate that he's currently wearing, wherever he happens to be standing. Then he goes to where the new breastplate is, and puts it on.
It gets even more chaotic when you have multiple dwarves in a squad, and you start churning out several breastplates. There's a hand-me-down cascade, in which the first member of the squad gets the best one, so he discards the one he's got on, and then the second dwarf in the squad claims
that one, and discards his current one, etc.