Most hybrids on the road are parallel and have a mechanical connection to wheels and vary rpm as needed, although the CVT implementations may hold rpm due to gear ratios like any CVT hybrid or not.

A direct mechanical connection is more efficient at highway cruise speeds than a mechanical->electric->mechanical conversion.

The main win a gasoline hybrid has is in running the Atkinson cycle gaining efficiency while losing torque which the electric motor makes up. This brings the gasoline engine up into diesel efficiency territory.

This is also why you don't really diesel hybrids, the engine is already very efficient but it is more expensive and heavier and hybrid adds more expense and weight.