Well it works both ways.
For my 3U assessment, I got 39/46. It was weighted 15%. So converting it, I got 12.7/15. This is rounded up to 13/15. Conversely, my friend who got 41/46, got 13.3/15. So this is rounded down to 13.
Theoretically, anyway. It might not actually happen to this, but when people talk about weightings and whatev, it would be a bit retarded to say, "oh, I got 12.423423423423432423/15 weighted."
I'm sure they don't weight your assessments until they need to send your ranks to the BOS, so there's nothing to fret about.