Yeah, it is due to the distribution of mass by stellar forge, it calculates star systems in "boxels" (cubes of space) - so you get boxels of similar-density systems and then also groups of boxels that are themselves of similar density (a sector is a large cube made up of lots of boxels).
Unfortunately this has resulted in very defined cubes of stars near the galaxy's centre.
[Frontier calls these subsectors, by the way: the boxel is just a fan term that stuck somehow. Probably because it's shorter.] - to quote @marx
There
was is a very detailed (and actually interesting) discovery scanner on the youtube
but it seems not to want to play, the link is:
Source: https://www.youtube.com/watch?v=Vz3nhCykZNw
EDIT: Seems it will play for me now (imbedded at least).
In lieu of the video I found a thread created by
@marx that covers some of this (and a whole lot more):
This topic seems to come up more and more often lately, so I thought I'd write another guide. Note that this will follow a more practical approach, mostly focused on explorers who heard about boxels and boxels surveys, and it won't go into detail about how sectors and boxel positions are...
forums.frontier.co.uk
(Well worth a bookmark I think.)