Brian S, 6 mV/Pa and 8 mV/Pa are perfectly normal sensitivity values for condenser microphones, and you're reading that chart correctly; between 6 mV and 8 mV the difference is only 2.5 dB, not 25 dB! So something basic is going on which we haven't put our collective fingers on yet.
The original poster has a pair of each type of microphone, no? If the left and right microphones in each of his pairs match each other reasonably well, wouldn't it be a rather huge coincidence if some defect in both microphones was causing an identical 20+ dB change in their sensitivity? If such a thing was happening to only one microphone of the pair, I'm pretty sure he'd be aware of that and would mention it, no?
By any chance does either of these types of microphone come in different forms or models with different sensitivity ratings? Might he be using a powering arrangement or accessories with circuitry which either amplify or pad down the signals coming from the microphone capsules? If so, then perhaps in the particular arrangement or form or model that's being used, the sensitivity is different from the nominal values that were quoted here.
I think, for example, that some people on this board use "battery boxes" which also provide some voltage gain, like a preamp. If that type of circuitry is being used, the manufacturer's specifications go out the window.
--best regards