Req. Rarity

Is there an equal chance for a weapon to drop q9, 10, 11, 12, 13 from monsters, chests, zchest etc.? Or does rarity scale with req.?

higher reqs tend to drop from higher level foes
lower reqs tend to drop from lower level foes

the problem is, monsters below lvl24 don't drop very many max weapons. At lvl24, the average req is between 10 and 11.

From level 24+ sources (including almost all high-end chests) the chances are exactly the same for a Gold weapon to be q9, q10, q11, q12 or q13. Before Nightfall release the chance for a q8 gold/purple weapon from those sources was exactly the same as any other req (9-13), despite what many players think.
For lower level sources and/or other rarity weapons the chances for differen't reqs aren't equal.