Well fold 3 didn't group them that way...that is how they are grouped on the NARA microfilms aka CSRs. All CSRs are in numerical order starting with the 1sts(infantry, cavalry, etc) many times the units on same roll have absolutely nothing to do with each other. I have found that the transcribers sometimes got confused when there were very similar names and got the wrong one on their card. As Bryan said, many men gave their old original unit for any number of reasons...unit pride, to throw off and confuse the enemy etc. I bet Bryan is on the correct trail...just like an old hound dog!!