The AI gets no bonuses. Shield mitigation increases as the ship takes damage, its intended to somewhat discourage focus fire (it really doesn't do a good job, so keep focus firing on important ships). Once it stops taking damage it will drop back down to 15%
Everything has shield mitigation of at least 15% at all times, but there are techs and abilities that can increase or decrease this amount (advent especially have a lot of them). Frigates will cap out at 57%-60% mitigation. Capitalships and titans have a bit higher max mitigation and it increases as they gain levels, so 65% mitigation is perfectly reasonable.
