CW, you asked the question, I was offering my explanation. Sometimes we work with the moulds we have and it is necessary to size down incrementally (yes, right after casting or baking while they're soft like you said!) to preserve concentricity and certain features of the bullet. Sure, you can whang a coated bullet through a hole .010" smaller, but does what you end up with shoot as well as it could? Maybe it doesn't matter, maybe it does; your gun will tell you.
There's no right or wrong way to prepare powder-coated bullets if you are satisfied with the results you're getting, and I think that is THE most important point here. I know you can't see my signature line on your phone, so I'll repeat it here: "It ain't HOW you do it, it's THE WAY you do it".