"Yellow tag" and my A&P's inspection

Denny wrote:
pipe to line up... What the hey? Gets out a strong light (why do all
mechanics work in a shop that is dimmer than a confessional booth?) and
takes a look... The head was re-installed on the barrel ten degrees out
of alignment...

Out of curiosity, how does the head go on ten degrees (approximately
or exactly) out of alignment? I thought the bolts or studs would
accurately locate the head and jug every time, so obviously there is
something I am missing or don't know.