Workflow purchasing group "validators" determination

Dear Gurus,

I have a workflow question. I recently created a purchase order with a purchasing group (PG1) needing validation to move the PO forward.

I snooped into the workflow through the service objects toolbox in ME23N. From there I clicked on workflow agents information as follows:

It displayed a list of 4 "validators" names all belonging to the PG2 purchasing group, the one that validates purchase orders. However, I can of course validate the purchase order with any other user that belongs to the PG2 purchasing group.

I validated the PO with my own user, which is different than any of the 4 displayed above and it worked.

So I'm simply wondering what this list is about and how it is formed. If any PG2 user can validate the order created by a PG1 user, then why are those specific users displayed in the agents information section of the workflow? A client recently asked me this and I had no clue.

Thanks!