That's where they were recruited, right?
Since I'm doing some research right now on a 54th Mass recruit who enlisted in Massachusetts but was born in Virginia, and therefore could count as a southerner, I wonder how the totals would look if sorted by birthplace.
The 54th Mass. was a Volunteer, not a USCI unit, so generalized USCT figures may not apply equally to them.. While a few were of southern birth, the great majority of the men in the regiment were recruited in the North, largely in the midwest (Mass. sent recruiting agents all across the country). I don't believe any of them came directly from the south, those southern-born had gone north before their enlistment. The same goes for its sister-regiment, 55th Mass.