The D1 D- is higher than the D2 A+. I don't believe it is a huge margin, maybe a letter grade at D2 to D1, probably less from D3 to D2. That's just a rough guess.
There used to be a glitch where a lower division school could move "up" from A+ to D-, meaning that they had moved from, say, a D3 A+ to a D2 D- in their grade. A fix was put in place fairly quickly, but that at least told us that they weren't actually that far apart.