smokepole,
Another difference is that custom reamers are often made of harder steel, so they don't wear as quickly. On of the gunsmiths I know had chambered a few hundred .300 Winchester Magnums with the same high-quality reamer, and hasn't seen any measureable wear. As a result a custom reamer can be made to cut a SAAMI minimum chamber.
Factory reamers tend to wear more (or at least those used in many factories) so they're typically made to maximum specifications, then used only until they cut a minimum SAAMI chamber. This is one of several reasons factory rifles can vary more in accuracy.
