There is a difference of .024. That is very little, so I don't think it has anything to do with the structural strength. Possibly the size of the raw material they start with??
That would be my guess. I have done a couple of subcontracts for DoD prime contractors. The military comes up with specs that sometimes make you step back and scratch your head. When you start digging, sometimes you find that the spec is completely arbitrary and there is no justification. Sometimes, the reason is an academic in nature (there are a lot of tenured professor wannabes working for DoD contractors). The other reason is that DoD contractors make more money when they don't use off-the-shelf components; they can pad their costs on a cost plus contract and that translates to bottom line profit.