Initially, it was thought the burial site on Ridgeway Hill, near Weymouth, dated from the Iron Age (from BC 800) to early Roman times (from AD 43).
But radio carbon dating shows the male rib cages, skulls and leg bones date between AD 890 and AD 1030.
Experts made the earlier estimate after examining pottery found in the pit.
Oxford archaeologists discovered the skeletons during the earthwork operation for the Weymouth Relief Road.
They have been excavating since early June.