There are a whole mess of reasons that this could be happening. Many of which have been pointed out so far. Here are a list of things that I have noticed in the past...
1. Firing all guns at the same time can be misleading as to which bullets are really hitting. This in and of itself is highly variable driven (convergence, plane attitude, G's, different gun loadouts).
2. Netlag, V-Sync, Lost Packets, both players connect quality in general... all contributing to Rubber Bullet Syndrome. This I've had some realllly bad experiences with myself. I dove down on a Lanc in a 262, tore him completely up (only 30mm, so they -must- have been the cause of the hit sprites), and they guy had -ZERO- damage.
3. If you knocked out all 4 engines, you must have been putting bullets all over the plane. Bombers can be bullet sponges unless you concentrate the fire in one place, and specifically in one vulnerable place (ie- cockpit, wings, tail).
I'm sure there could be some more reasons as to why it happened... some explainable, some unexplainable.
Hope this helps some,
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