In theory, damage are simulatenous. What is possible is that your damage were allocated to an "invisible" unit, eg AT or militia that was hidden from you. It may explain why your bomber died immediately : 6 or 8 high level militia can add up in terms of AA damage.

Attacking a stack with a single unit is never a good idea.
In the case of the plane warfare, the defender get's to shoot first. As AA has a high AA value and most infantry units too, note that militia are stealth. You can receive a lot of damage. Once the enemy has defended it is to you to attack the enemy ground units. But in your case your plane is so damaged in the proces that it dies instantly.

Cruisers are the AA of the seas. You should never attack cruisers with naval bombers as it will be a very one sided battle. If your plane is on patrol then it will attack every 15 minutes. If a cruiser is in your patrol circle than the cruiser will deal defensive damage your naval bomber.

When a plane attacks, first the target deals defensive damage to the plane, and only then does the plane deal damage to the target. If the target's damage harms your plane, you will end up dealing less damage, and if the plane gets killed, you don't do any damage at all. AA is not in any way specific, it just has higher damage values against planes. On this note, ALL units have defensive damage against planes (even truck convoys actually). But, those damage values are smaller, so they don't one-shot the plane that often.

If army A fights a 2x smaller army B, army A does 2x more damage than B every round of fire. This meaning, B's health goes down 2x faster than that of A, and also, B has 2x less health. So, in the end, A wins with as much as 3/4 health left. This means, you should ALWAYS seek fights where the enemy army is far weaker than yours. Planes are fast and independent on ground, so you can easily stack them into large groups and destroy all the small forces your enemy has, even if those are hidden behind the front. This is why planes are so glorified in this game - they can take numbers advantage and vaporize the enemy forces, if you're smart enough and your enemy isn't on a very high skill/activity level.
This meaning, attacking a large stack of units with a single plane basically reverses its advantage and makes the plane VERY ineffective. Instead, try to attack a single (or few) units with a large stack of planes, so the advantage is on your side.