My experience is too fuzzy to go either way on this. IE the "hints" of something fishy are in the noise. Noise IE anecdotal recollection's fuzziness.
Relative fighter:bomber tater damage seems about right. One of the big deals with doing damage on bombers is the relative scales. If you tell yourself I put 3 taters within "1/5th wingspan" of the wingroot, that's a big difference in absolute distance depending on whether the target is a B24 or an La7. I honestly don't have any trouble taking down bombers with german 30mm if I don't screw up - if the tater burst lands all in the same spot.
If I put 3 taters in one area (say wingroot or mid-wing or within a 5m diameter on fuselage) and get no kill, it's pretty much guaranteed (again this is from experience, no actual knowledge of the damage model brass tacks) a second pass with 20mm hits on that same target area will kill the bomber.
I flat out disagree if you're saying a bomber's (e.g. A20 or B17) structure is no stronger than a fighter's equivalent -- apples to apples like an A20's main wingspar element compared to a Hurricane's equivalent.
An A20 can take 5+ hits, but exactly where those hits are, what the internet was like, etc, are all non negligible factors. Saying 5+ hits isn't enough. But all things being equal, 5 hits versus 2 sounds about in the ballpark. IE as it should be. In my experience 2 hits for fighters is exceptional. Most die to 1 hit.