Some do, some don't. Even for one company, it can vary. I can't think of a recent ball that's different at 14# from the 15# or 16# version. There are always slight differences in RG and diff, but the 14# is a "real" ball, across the board.
Brunswick seems to put out 12-13# "real" balls. Some Lane 1 balls are available in 13#. For the smaller guys, like Lane 1, DT, or RG, if there's a 13# version, you can pretty much count on it being a normal ball. Track uses the Crunch Time core in a lot of their lighter weight balls; it's still a real weight block, just different. Columbia and Ebonite use some lightbulbs in their 13# balls; again, a real weight block, just not the same as the rest.
SH