For the original question... unless the forgings are mated together before machining, drilled out together, and then hand fitted, you will have lines that are a little off.

Since the uppers and lowers are machined separately and pulled from a parts bin, there will be slight differences... like the holes being drilled for the pins and they are each a thousandth's of an inch of center in opposite directions on the upper and the lower... It could still be in spec, but cause the alignment to be off ever so slightly.

This is one of those things like slop between an upper and lower... doesn't affect accuracy or reliability, so it's a non-issue.