The "reference" density is 160 dpi, do you remember Android phones with 3.2" screens, running at 320*480 pixels?
When you have triple the density at 480 dots per inch, and 1080 pixels (dots), recalculating to 160 dots per inch gives:
1080 * 160 / 480 = 360 logical units, which is pretty close to 320 logical units on those old 3.2" screens (and this is not a simple coincidence).
Another example: 1280 * 720 at 320 dpi, e.g. the Galaxy Nexus.
720 * 160 / 320 = 360 logical units again, and again, it's not a coincidence.
So screens on Android *phones*, over time, keep having pretty much the same logical size. This is important, since all layout (where things are relative to one another) is done in logical units, and more pixels just makes things more sharp.
At 380 dpi, your phone's screen width was 1080 * 160 / 380 = 454 logical units. Too little for a tablet (those start at 600), but unusually large for a phone class device.
Now at 360 dpi, it's 1080 * 160 / 360 = 480. As it turns out, that's the smallest size at which two panel layout is enabled in Aqua.
( BTW - when working on graphics, I have to do these conversions all the time, and not with nice round numbers... )