Do you have a specific example, maybe with screenshots? You're combining a lot of terms in confusing ways.

SDC is not required to upfare (although maybe what you mean is that the differential to buy-up is lower on a different set of flights so you SDC from what you booked onto that other set of flights and then do the cheaper buy-up)

Google Flights doesn't even show fare classes in its search results, so I suspect you're using that phrase to refer to something else. If you just mean that the exact same flights are cheaper in first/biz on GF than on the Delta web site, sometimes Google is showing cached inventory.