If I understand what you're asking, here's my answer:
FACT: A Mode C altitude broadcast is never corrected for changes in pressure at the aircraft. It is always broadcast as if the barometric pressure is 29.92, and the ATC screen then does any correcting on THEIR end.
My ASSUMPTION: That's why ADS-B also broadcasts an uncorrected altitude (29.92) - so that ADS-B OUT and Mode C traffic altitudes can be apples-to-apples once corrected on ATC's screen. Since uncertified ADS-B IN systems don't typically have access to pressure altitude, they can only tell you their own geometric altitude and thus are prone to fairly large discrepencies (i.e. 600ft) vs certified Mode C and ADS-B systems.
I see what you're saying - since both altitudes are available from the ADS-B broadcast, why not use the geometric altitude for the comparison... good question. There is nothing in the airspace system that is measured or dependent on the geometric geoid thingy, so maybe the GPS geometric altitude is only broadcast so that the FAA can verify the integrity of the GPS altitude report vs pressure altitude report. Or perhaps there's a standard in the ADS-B world that says always use the pressure altitude, since it would keep the apples-to-apples integrity intact on advanced systems that also display Mode C and Mode S targets. I imagine a homebuilt Stratux box could be easily reprogrammed to use geometric altitude, but then it would be the only one in the game doing so.
Disclaimer: My info comes from The Google so it must be true. might be wrong. ;)