Originally Posted by
Atticus Batman
That has to di with your ego level. Once you get to a certain level, most weapons you get have all slots open and a nano, the around Ego 3200, it resets and you go back to getting no nano guns with one slot open, and they slowly open more as you level again. Sometime after ego 3800, you once again start getting guns with 3 or 4 open slots and, a decent ammount of nano-guns.
EDIT: I see Chev beat me, but seeing as I am ego 3700 I am going through what he mentioned, right now. At 3700 I just started getting nano guns, and guns with 3 open slots again, as well as the mod synergies.