I had an RCBS trimmer for many years, but decided to make a change about 3 years ago; it was tough to keep adjusting the trimmer in order to turn out identical length cases in a large batch. I bought a Hornady trimmer, and it is a very good tool. I do check with a 1/10th mike every 10th round as a "failsafe" mechanism. I can run 100 rds., check each of the first 5, then one out of 10 for the rest, and it will hold within +/-0.001" without further adjustment.
What I DO see is cases out of "true" with the rim, causing the 0.002" min-max variation. Usually, if I mike the case at 2 or 3 places on the case mouth I can see that the rim is slightly out-of-true with the case centerline, and the length will vary measured at several points around the circumference by +/-.001". The brass is stressed between the shellholder and the case mouth pilot, and will flex a little during turning.
There is a small O-ring in the trimmer on the shaft that seats the rim into the shellholder, and it does need to be replaced from time to time. Fortunately, it's a hardware store item, and about $0.40 for a bag of 2 rings - so I keep an inventory of a few on hand.