Ideological Incompatibility

Alright so can anyone explain what the numbers mean for ideological incompability and how they're calculated. The way the numbers are calculated seems arbitrary and sometimes even outright wrong.
For example in the first two screenshots a +5 and two -3's equals a -5 but in the next screenshot a +10 and two -3's equals a -6. And then in the 3rd screenshot it tells me census suffrage should be a -3 as an option but as my active law it's actually a -5.