I didn't say "clearance", I said "1" between stated barrel length and stated rail length". So an 11.5" barrel would typically require a "10.5" rail, except that there aren't many (any?) 10.5 rails so you wind up with a 10.0 being the longest you can use.

There are exceptions, especially since there's no real standard (other than 7.0, 9.0 and 12.0 rails being supposed to fit between upper and FSB on carbine, mid, and rifle length gas systems respectively) and rail lengths tend to vary from manufacturer to manufacturer. Hell I had a DD 9.5 and 10.0 and they were virtually identical in mounted length.