I saw something on another thread on this board about 16 rough being done in the 30's and 40's... I suppose that the other possibility is that after the rough was done, during the framing of the condo they moved the wall slightly for some reason... And it's not the space behind that's the issue - it's that the current 12 toilet on a 16 rough doesn't let the small bathroom door open fully.
Rick
In the 20's with tanks that mounted on walls, they used a 14" rough.
There never was a 16" unless the framing was missed.
As plumbers, we shoot for 12"
Sometimes because wood is in the way, we wind up at 10" or 14", or close.
On a concrete pour, things can get knocked around, or they might know exactly where walls will be framed.
Often, the lower floor was left unfinished, and it was years later before someone decided to layout a room.