Speculation wrt to the suppressed star types along the X=0 and Z=0 was/is twofold:
a) there are sufficient manually authored systems around the bubble, so the FD's plan was to suppress certain proc-gen star types in the area around the bubble
b) while programming the suppression of the two axis an "OR" was used instead of an "AND", resulting in two suppressed lanes instead of one suppressed area
Huh, that's actually the best explanation I've seen yet for the empty bands across the galaxy.
This thread is awesome by the way, the maps are great to look over.