If I understand correctly tieflings are normally the children of a humanoid and a devil, but the grove tieflings are not. I think they became tieflings by simply being "teleported" (magicked?) to Avernus by a demonic plot, which magically turned them into tieflings while they stayed there in the first layer of hell. Not sure I got that right entirely, but it was the entire city of (can't remember its name) that got magicked to hell. Meaning mostly just ordinary citizens, men, women and children.
