They don't only. They are just most common.

I have the same problem the most dropps are small, large bottles are very rare.

Superior potions do drop in stage 15. It's just RNG. Sometimes you get lots of superior potions in a couple runs, other times you don't. 

Been farming Keeps ,, Dumped over 3000 Evenergy into them in last coulpe days  ,,,,

3 Superior Pots .................

Dumped 1200 into Force Keep today ,,,, I got ZERO Superiors ..................

this just tells me the Game is saying Blatantly

***, GOOD LUCK,, You should JUST BUY SILVER PACKS and Combine large ones ***

Seriously Flawed ...............

and especially with the Arcane Jeep, I don't get super bottles with the last level

i

this is the problem I will never get, I have never achieved such a super bottle. 

just those fucking little bottles. In the last level you should get more chance for super bottles.

So I'm stuck here for a while