This was discussed on the discord some months ago, but I can't recall if a definitive answer was given. So I'll pose the question again here:
I have previously thought that a half-orc was always the result of a orc and human coupling, but both IC lectures and the NPC descriptions in the half-breed camp indicate that half-orcs in many cases breed true: A half-orc mating with any other humanoid will usually produce a half-orc (or full orc if horc-orc).
The IC lecture attributed this to the "fact" that "The blood of Gruumsh runs strong", and I find that a rather interesting narrative to follow.
Tldr: Will a horc-humanoid coupling usually produce a horc because Gruumsh Stronk, or are all horcs the result of a (usually) tragic circumstance?