Running El Capitan, I have two external monitors and the MBP monitor, three total. When the MBP is not hooked to any monitors, I have four Spaces set up, each with their own desktop background (A, B, C & D). When the external monitors are hooked up, I get two more spaces ("Displays have separate Spaces" is checked) with two more background pictures (E & F). I arrange them in a V:
Ext1 -- Ext2 \\ // MBP
The top left monitor becomes my main display (Desktop #1) and has the four spaces. I don't know why it does this but it does, and that's what I want, too. So it has desktop backgrounds A-D. Top right gets picture E (Desktop #2), and the MBP gets picture F (Desktop #3).
This is exactly what I want, as A & E complement one another. Except sometimes (maybe 3% of the time) the MBP becomes Desktop #2 and gets background E and the upper right becomes Desktop #3 and gets background F, which is just visually jarring to me.
When it happens, everything is still arranged in the V that I want, so it's not like the OS is confusing what's what.
Q: Why does this happen or how can I fix it?