Thanks Micah! You've solved one of the mysteries of the game for me! From now on, I'll definitely stop allowing tax to fall to 90% when my capital has 1 or 2 unrest.
Edit: So, I looked through all my games, to see where I could optimize by forcing tax to 100 where it's currently at 90 because of 1 or 2 unrest ... and they are all negative dominion provinces. Bah
. At least I understand how this works now.