Not sure if auto assembly lines do the actual balancing; my guess is that it's subcontracted out. The rims were most likely sent off to a large tire shop, balanced, then sent over to the assembly line. With literally hundreds of tires balanced a day, it's very possible a few do not get balanced perfectly.
Other vibrations in tires can be from a belt seperating in a tire, although this is usually felt at lower speeds. Another is a tire that's out of round; that's rare these days too, but could be the culprit.
Take it in, get the tires re-balanced.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.