At this stage... I'm starting to find the sheer number of different things baffling.

But everything ties together.
Can you give me an idea of the scope of your search (like how many mass code D boxels you are looking at, are all the boxels contiguous, and are you scanning every single star system within the boxel, and are you uploading to edsm?) If you were to scan a set of neighbouring mass code D boxels in their entirety, it would be possible to extract the position data from edsm and plot them to see how the clumping looks. Or if you were to scan a column of mass code D boxels and record the masses and ages of the stars, we could look at how that varies with height above the plane, and if you note the number of stars within the boxel, we could look at the scale height as it specifically relates to mass code D stars - it's possible that each mass code has a scale height?
Ooh.
Now that
is a thought, and ties to what Satsuma mentioned above and what others have alluded to - we know that certain mass codes drop out in different places.
(edited to add)
I am confused and baffled, but I'm having fun.

And thanks to all of you and to also kudos to edsm and eddiscovery!