I heard rumors about skin rerolls being weighted, so I decided to do a test using the free RP the PBE gives you. You're limited to a certain number of chest purchases a day, so these are the results I got in a little over a week in purchases.
Using skin numbers from the wiki, 1350RP skins make up about 25% of all skins. In ~50 skin rerolls on the PBE, I receive _zero_ 1350RP skins. I received a disproportionately large amount of 390/520 skins.
I was extremely surprised to receive Zombie Brand. Could it be possible that the skin reroll system currently on the PBE just doesn't let you reroll into 1350RP skins? I have no idea what the difference between the reroll system on PBE and live is, and I could find no Riot official source on how exactly the skin reroll system is intended to give skins.