To OP, one other thing to consider is the rim & tire weight. On an loaded test spin a wheel with a heavier rim and/or tire will spin longer.

But unloaded spin tests are pretty much like kicking the tires on a new car. Even if the rotating weight is the same and you are detecting the difference in bearing/seal resistance it will be a small fraction of the total resistance encountered when actually riding and will be virtually undetectable in regards to performance.

And as mentioned above, the tighter seals and freshly packed bearings actually should roll slower that the same hubs would when have the grease is gone and the bearings are looser.