Although I have no experience grafting cannabis I do have a few years experience growing citrus (over 40). Where this may relate to your question is the use of different rootstocks and different soil conditions. First since you have such a limited sample (very few plants) it will be hard to tell what the different rootstock effect will be on the grafted portion of the plant. It might be something as simple as a better match with the cambium and thus a healthier plant from day one. But in my experience different rootstocks will react very differently to subtle changes in soil, moisture, nutrients, and any number of other conditions. Even within the same rootstock you will see different growth tendencies for no apparent reason. I have also seen plants do some really strange things as the damage or shock from the graft actually mutates the plant is some way. It is a shock to a plant to graft it and they all don't react the same way. Mother nature does some strange things and what we do to the plants just makes it worse.
It might be interesting to continue to graft to an indica rootstock and see if you can duplicate the results. In our neck of the woods the sour orange rootstock has long been the choice due to fruit quality, yield and other factors but is one of the weakest when it comes to a devastating citrus disease that kill the whole tree in a year or two. So we have planted for years on other rootstocks but still come back to the tried and true of the sour orange. Just plain works.
But since you have experience in grafting and were successful with your grafts I am betting you knew all of the above.
One other note when I was growing up we would use the plastic bread bags and cut them up to use as our grafting tape. We didn't graft citrus but avocados are always grafted. Same scion to the same rootstock, that would give us mature fruiting wood rather than waiting to see if the plant would bear fruit. Grafting is an art and requires some skill to do it right all the time.