Like I posted each battle has it's own difficulty level. It's random as to how hard the battle may be. The difficulties don't swing wildly, but they do enough to cause things to not work as anticipated.
It could be anything like...
1) AI strength matches player strength + 20% accuracy
2) AI has 15% more relic boost per relic than player + 15% accuracy boost
3 ) AI has 50% more relic boost per relic than player + 15% accuracy boost
There's no telling.. but fight the SAME battle with the SAME characters and the SAME talismans enough times and it's clear that the AI teams are changing in strength to a degree, above and beyond any accuracy boost. Not enough that it's readily apparent. But enough to possibly cause a loss every now and then.
The skull vault seems to be the biggest battle doing this, or with the largest difficulty swing. I can auto it for days.. then lose, sometimes more than once. There's no other cause for the loss than the fact that the battle itself has boosted the AI team in some way - made them stronger, made them smarter, something.