You are quite correct. Seth P. Andrews is not listed in the roster of William D. Henderson's book on the 12th Virginia Infantry. I have a second edition copy which has an addendum with additional names, but ... no Seth Andrews. However, he has a Compiled Service Record with that unit. You may wish to order copies of Andrews' records in the 12th and the 9th Virginia Infantries at:
http://history-sites.com/research/index.html

How did he "slip through the cracks"? Hard to say, but, as someone who has spent hours upon hours watching microfilm go by...it takes only a second or two of inattention to make a mistake. It's unfortunate that only a handful of the Virginia Regimental Histories Series writers ever participate on this message board.