According to Dr Andy Foote from the University of Aberdeen:
We found that one form, which we call 'type 1' had severely worn teeth in all adult specimens.
The other form, 'type 2', had virtually no tooth wear even in the largest adults
It seems that type 1 is a generalist feeder, consuming fish and seals whilst type 2 is a specialist feeder that scientists suspect exclusively feeds on marine mammals such as small dolphins and whales.
Genetics indicate that the two types come from two different populations.