I don't know if it's still the case, but it used to be in Virginia that it was the buyer's responsibility to notify the DMV; the seller signed and handed over the title; if the buyer never registered the car it would be completely possible for the DMV to not have record of the transfer in that case.
While not required, the seller can also mail back their registration with a notice of sale (there is a small form on the back of the registration card, if I remember correctly).