My guess is that it is based strictly on your inventory of troops. As you level and trait your troops, your hero power increases and your PVP results decrease. I’ve noticed with each new kingdom that comes out, my maximum PVP scores seem to decrease. If you’re interested in testing this, one possibility might be to disenchant all copies of all the common and rare troops that you don’t regularly use to reduce your inventory of available troops. This might decrease your hero power in a meaningful way but it might also do nothing since the game remembers the level and traits of any troops that you fully disenchant.
I personally tested the “move to a lower” guild strategy a little over a year ago and documented my results in a forum thread. That alone was surprsingly effective, raising my potential points from around 55 to 66 per battle (if I remember correctly). I’ll see if I can find those threads…
Edit: here is a link to the thread with some good discussion and data.
And here is a link to a discussion with even better data: