How the heck does cruiser AA work

I've just done an air strike by 3 stealth bombers (heavy) on a city containing 1 L3 SAM (9 damage f/w) and 5 L2 SFs (2 damage each vs f/w) patrolling above. To my understanding, my bombers should have received around 19 damage on average, because they shouldn't be affected by AA envelopes and so only receive Point Defence (each bomber has 26HP, so I thought it'd be a safe-ish shot). However, they actually received 41 damage, and I lost a plane. Bit disappointed, but I'm assuming it's got to be bad RNG? There's nothing else that could've boosted the AA's firepower?

Bombers are unable to see stealth ASF, so they might be a possible though unlikely option

Combat in general has a lot of oddities, mainly cause by the lack of combat reports and therefore a lack of ability to control if everything works as intended.

Like air assaulting Spec forces getting shot down by ships, who should not see them on their radar

It definitely wasn't that, because he conventional ASFs are only L2, so he can't have got any stealths.

I think it must have been another SAM unit. This guy has SAMs in all him homeland cities, but none of them were inside the radius of the one I attacked. However, one of them is close enough that the bombers' patrol range would have just about overlapped the AA envelope of a neighbouring city as it delivered the strike - even though it was a direct attack and not a patrol order. I guess this must have triggered the neighbouring SAM, though I think that it probably shouldn't.