I have a couple of thoughts. They really don't help you nail your man down, but might be worth considering.
1. The lists you are looking at are "consolidated" lists. I take that to mean that they were drawn from more than one source, or perhaps multiple persons were going around trying to nail down all who were eligible for the draft. Since one of your men has "unknown" under his occupation, the information certainly didn't come directly from him. Perhaps that came from the landlord of the building, and he probably didn't know the exact age of Andrew and may have been guessing. Andrew himself may have provided the information that includes his occupation as a machinist. My guess is that these two listings are for the same man.
2. I agree that getting the right age on records (census, military, etc.) was not an exact science at this time. However, my feeling here is that none of the military records Michael came up with apply to this man. I believe heprobably did not serve (not all who had to register ended up in the military), and it may have had something to do with his occupation. He may have been exempted as an individual whose skills were needed in the production of war materiel. There were other ways to avoid the draft as this article shows.