I base my information from my time spent working at a Type 08 FFL, working on getting ATF variances, and dealing with various actual manufacturers (CMT et al) to get our own house-brand lowers.I've worked in manufacturing and engineering for several years (before and after military service) and Tamara's summary is FAIRLY accurate.
The CNC variances in companies who machine from 80% forgings come from when they change certain dimensions or from a lack of final QC.