That's a very good question to ask, Jeanne, that reminds me of the Nanotyrannus fiasco.
To answer your question, though, they believe that they are different varieties because each group has its own qualities. For example, one of the dorudons, ancalecetus, was much more streamlined than a regular dorudon. For a comparison,
ancalecetus
and dorudon
They may not be different species, that is a good point, Jeanne.
Onto another point, I have learned to dislike certain Evo-buzzwords: primitive, advanced, basal, early, terrestrial cetacean (land whale, oxymoron much?), non-avian dinosaurs, and other stupid terms. The evolutionists think they can find out everything about an animal just from its bones. Many of their buzzwords are ambiguous, so anything and everything could be considered primitive or advance. Modern whales have teeth that are the same, called homodont, but basilosaurids have heterodont or differentiated into canines, incisors, premolars, and molars. So why is the chewing, snake swimming, heterodont toothed, whale considered primitive to slurping, biting, homodont toothed whales?
It also reminds since the evolutionists took over the taxonomic system they can get to declare any animal apart of any evolutionary line. Baleen whales, or Mysticeti, have three different kinds of whales, Aetiocetids, Eomysticetids, and Mammalodontids had teeth but no baleen. Yet they are considered baleen whales, so much so, they even say that these had both baleen and teeth but there is no evidence of this. They even say they fed by suction of feeding, which started the growth of baleen, Really?