Do vibration motors vibrate in both directions at equal magnitude?
DC motors can turn faster in one direction than the other.  Basically, if the commutation is adjusted so that the brushes switch a bit sooner than the mechanically neutral position, the motor favors that direction.  ("Timing" DC motors used to be a hop-up trick for slot car racers, back when slot-car racing was a big thing).

Whether your motors do turn faster in one direction is based on the motor design and construction.  I could see a manufacturer doing that on purpose, if they had good enough process control (or I could see it happening by mistake, and persisting).