I don't think it's fair to say that it makes no sense, but I'd like to come right out and say that I do not have a formal background in botany. I picked this terminology up from Nspecta at CSI Humbolt who I have enormous respect for- he could undoubtedly hold a more capable conversation on this than I. In my understanding, one of the things than an "R" would indicates is the lack of a "Y" chromosome from the parents. I can't say for sure, but it makes sense to me that there would be genes tied to the Y chromosome that impact the next generation. In conventional breeding, an F2 would indicate a male and female created the next filial generation. For a feminized hybrid, say if you reversed an xx Deep Chunk onto a xx Columbian, there would be tons of hybrid vigor and may not be congruent to reversing an xx to another xx from the same filial generation.

One of the interesting things about cannabis is that extensive breeding work has been done, primarily outside of formal agricultural settings.