Interesting... I hadn't heard or thought about that!
It makes sense and helps us out a bit(vs what I had in my head), since we self-imposed down to 23 last year(before announcing penalties) that we also easily could have done the same with the overall roster(we were definitely under 83).
So, we both come up with the same 81 number(through very different methods). Seems likely to be the correct number to me...