Catch 3 Poison Type Research Task

This is one of my favorite research tasks. I didn't have a Muk yet (at all) and was able to evolve a good IV version and am now grinding to power it up.

But here's my question. I've done this Task 8 times now and every time it has given me an encounter with Grimer. Given RNG could be occurring, But are we really sure that there are other possible encounters? Has anyone else here done this task and encountered a Gastly or a Bulbasaur?

Every Pokemon site I've seen lists three possible encounters of this task. But from what I've seen all of the Catch 3X of a type Research Tasks only have a single possible encounter. Why would this one be different?

Just having doubts about the information being supplied. 151 Tasks completed, so far.