AI more likely to give up three provinces than to accept a humiliation?

[Wizzington]

What I find odd is that the calculation does not appear to be the same when the player offers peace to the AI and when the AI evaluates whether it should offer peace.

This is easily apparent when annexing an unciv: although it's a 85% warscore peace cost, the AI will only ever accept a player-proposed peace at 100% warscore if there's an annex goal. However, the AI frequently offers to submit to the annexation at 85-90% warscore...

The calculations are actually identical as of HoD. What's happening is likely that some factor in the calc changes between you offering and them offering.