Bro. Jeff, I'll try to look when I get home, and see what I have on the Union Baptists. I would expect the Old Regulars are undercounted, but, IMO, probably not as undercounted as Primitives, based on the simple fact that there are more Primitive Baptists than Old Regulars. The count for the 17 associations in the study is accurate, because it is based on the records of those associations. The question, though, is just how many other associations and churches there are out there we don't know about.
Bro. David, the numbers on adherents.com is from the same source as I gave above - Churches and Church Membership in the United States 1990. The difference is that the number I gave is from the "baptized members" column and their number is from the "adherents" column. The study creates these two different columns of numbers for the purpose of comparisons between denominations. Membership in denominations that practice infant baptism and those that practice adult baptism only are not of the same type for comparison purposes. Baptists, and especially the "primitivistic" ones, will have a lower number because of people "involved", attending, etc. but not actually members. The "adherents" column trys to bridge that gap by accounting for the children and others who are similarly attending Baptist services but not counted by Baptists, while other groups are counting them as part of their church. This off the cuff non-scholarly explanation may make absolutely no sense. I'll try to look up the way the study explains it and post that later.
I also have a "guess" about the Sovereign Grace Association. I have never seen that name in anything about Old Regular Baptists. That makes me assume that they are a new association (but they could be just an old "undiscovered" body). Assuming they are new, I would guess that they have separated from the other Old Regular Baptists to follow a stronger "Calvinistic" position (based on the usual emphasis of the term Sovereign Grace). Just something to think about, but totally guesswork.