I guess that makes sense, since the - output can't really go negative. It stands to reason that it rides on a bias voltage. You can probably tell I don't have allot of experience with differential output sensors nor do I use op-amps like this normally. Thanks for mentioning that.
That firms also sells the same pressure sensors with active electronics already installed inside and it outputs as I recall a 0-5vdc measurement range and it's temperature compensated to boot. Cost more but now you can probably appreciate why that is so.