Are merc spawn rates completely random?

I by now have farmed over 700 (strand) maps, probably more and been looking for a KB Merc with return since league start. I have found 1. On a 3-link. During my farm it felt like I get 3 Sanguimancer on 1 KB merc.

I do have 2 KB mercs in total. They do no influence the spawn rates of new KB mercs, right?

Also: Why is return obviously weigted into oblivion? If I wanted I could do 40/40 today and would not have been able to play a build, which will probably never return again. Even though I already put DAYS into searching for this stupid merc.