There is likely a joist in the way to have it centered at exactly 12". Those are all within tolerances.

I'm not a plumbing expert, but I just read up so I could pick my toilet recently. Is it possible you have a 14 in rough in and the plumber just assumed it was 12 when he told you 12? It's possible, 12 is overwhelmingly the most common. You may be able to get a 14 in unifit adapter from Toto if that's the case. I don't know how they work in terms of if they are rigid or more telescoping for length, but if they have some flexibility in length the 14 in might allow a good fit.

I'd at least ask, as that would be an easy fix if that's the issue.

I bet kirkhall is correct. We also ran into a floor joist problem. You can try the unifit adapter, but if the 2" is actually slightly less than 2" it probably will not work as my understanding is you can only make changes in 2" increments.