Fully extended is one set over none extended: base ten.
It makes sense if you think of all of them extended as a set, rather than one less than a set. And so by the time positional notation came around, it was cemented that all of them extended was a set. (It took us a while to get 0 and positional notation.)
Just look at Roman numerals:
A finger. A hand. Both hands. A hand representing both hands. Both hands representing both hands. Etc.
It makes sense if you think of all of them extended as a set, rather than one less than a set. And so by the time positional notation came around, it was cemented that all of them extended was a set. (It took us a while to get 0 and positional notation.)
Just look at Roman numerals:
A finger. A hand. Both hands. A hand representing both hands. Both hands representing both hands. Etc.