[ger21]

That would bring up the obvious question, what if you actually want a hole .250"

That's what reamers are for.

 

[ger21]

Collets by design should allow for a slight variation of sizes. They are probably designed exactly to the specified size, but can accommodate undersize bits.

I have a bunch of Porter Cable routers, and I think their collets are slightly undersize, as bits won't usually slide out even when they are loose.