Were all of the local stores you looked at part of the same chain? Maybe it's possible the Mobil sales guy had a deal going on the blend?
Dunno what the reason, but they are two separate products and i have seen both being sold next to each other on more than one occasion.
But regarding the difference between Mobil Drive Clean (100% dino) and Superflo, they are different oils. Different specs, different properties, etc.
However, if you're topping off a Mobil 1 fill, you should use Mobil 1. I would doubt it, since they're the same manufacturer, but additive clash is possible and should be avoided if possible. Mobil 1 is really easy to find, so the excuse of availablity shouldn't be an option. I always have a quart of Mobil 1 in my trunk just in case i need to top up.
ferb!