Martin,
Well I can't explain it because I have never actually researched the numbers myself. I can however verify several sites online that deal with the subject and it indicates to me that serious research has been done. In another post I recommend Col. Mike Kelly of the 37th Texas as an expert, I am sure that he can give you some sort of factual numbers.
To clarify why I have never researched the numbers is I am not into using their service to justify The Cause. They served that is fine with me. I have already seen several times where it is said the Union could not have won without "us" so I can already see where the Confederate argument would lead.
Race is not mentioned on Confederate CSRs, or at least not that I have ever saw, so it is possible that a good number of slaves could have served under their masters names.
I believe Fredrick Douglas also mention black Confederates. That should be acceptable as verification.
GP