Here's what I found on S10Planet.com. I think the way it might work is that at idle, the MAF is used rather than the MAP, but at part throttle the MAP takes over. See what you can gather from this explanation:
lol, that looks familiar
The map sensor will still read though. It is just the degree to whitch it is used by the computer changes. Wot is all maf. Part throttle being a mix between the two.
Post a datalog in kpa. Also make sure the units are being read right, are you sure it is in-hg, and not psi? In-hg is aproximately psi*2 i think, the numbers start to make a little more sense.
I am not real positive why the numbers really matter though. I guess it depends on what you are trying to do. I do all my datalogs in kpa since that is what my tuning program uses. And i know the kpa readings are correct.