Michael,
It's a lot like @twocee says.....
Analytically, it's the numbers game. B-24s operate in bigger formations (very big in ETO, medium size in MTO, near squadron size in CBI and SWPA). All those gunners shooting at a few fast moving fighters; if one fighter starts to smoke (or just rolls over and accelerates....causing increase in exhaust smoke), 50+ gunners will claim a kill (and we're not talking about any dishonesty, just the real belief that you shoot at something, it smokes and looks like it's going down, so you claim a hit or a kill).
Navy PB4Y-1/-2s operate in small numbers, singles or pairs mostly on shipping patrols or ASW. Only occasionally in half-squadron numbers or more, for added numbers in actual bombing raids. The navy claims tend to be more accurately reported, as they have more time to see the result.
That's a generalisation, but you get the overall picture. Remember also, that for bomber formation gunners in ETO, the effective range for a 50-cal means the fighter target is about the size of your thumbnail or fingertip to first knuckle with your arm outstretched. The attacking fighter may be the size of your pinkie-finger nail when it starts lobbing 20mm or 30mm cannon shells at you. So the action mostly happens a bit further away.
From what I've read of the SWPA bomber ops, the Zeros came in close due to the slow firing nature of their 20mm cannons. Also a lot of the PB4Y kills occurred with them chasing the Japanese aircraft (including a few fighters!). So, I'm guessing much closer range, and hence better to observe actual results.
Thinking 'while walking & talking,
...geoff