You can probably try to make some rationalization on this, like 3072x1920 px is less than two times more pixels than the 2304x1440 pixels of the panel (actually 16/9 more pixels as it's a 4/3 larger resolution in each dimension), whereas 3360x2100 is more than two times more pixels to interpolate.
Or consider it's up to 4*1536*960*3*60/2^30 = 0.99 GB per second of RGB datas to render and then scale down to the screen resolution (1/60 of GB 60 times), while it's of course more than 1GB/s for 2x 1680x1050. That's at least just a round number, I don't know if it's at all meaningful regarding (Broadwell) Core M specs.
Or you can also relate the rMB to other retina Macs and see Apple has here to do weird things because they couldn't get a 2560x1600 screen for the rMB and has to use a lower resolution screen: if they did as usual with this 2304x1440 panel, the offered resolutions would have been HiDPI 2x (1024x640, ) 1152x720 as the default, something between 1296x810 and 1344x840, and the max at 1536x960.
But of course 1152x720 would have sucked as the default out of the box, and the rMBP13 having only a 2560x1600 screen they couldn't choose any higher resolution as the default for the rMB12. Well, still having hopes in two or three upgrades of the rMB, say late 2016 or 2017, to see Apple moving to a 2560x1600 screen for the rMB12 and give the rMBP line an upgrade to 2800x1800 and 3360x2100 all at ~250 ppi and setting as the default their current second best resolution like on the rMB.
As I was saying at post #12 in first page when I proposed to try this resolution, this is just the same ratio as the max resolution proposed on the two rMBP or the riMac, so you're not going further with the rMB panel than Apple support with its other retina Macs with similar pixel densities. And then, physical screen resolution is one aspect, but actual GPU capabilities in a fanless Broadwell Core M laptop is another. And I don't really see the point at trying to burn ones eyes with larger resolutions than 1440x900 or 1536x960 on a 12" screen.