Its just me, but I had the same bad luck there.
Still remember how I passed 100 at the end of october, then got 1 super kage scroll from it right next day. After that, got my 2nd super kage scroll on 120. Even now, I still check everyday, and 100 have dropped it only once after that. So overall, 11 weeks - 2 superkage scroll from exam 100. call it rng or bad luck, but this is really weird.
And yeah, you are right. We get a new type scroll every 20 level for sure, like at 40 , 60 , 80 etc. And after every level in between then(i.e 50-59 , 70-79, 90-100) any among those levels can drop the next level scroll.
For example- 100 drops a kage scroll from sure. 80 have a chance to give kage scroll, and any level between 90-99 can give kage scroll. That's how it worked for me till 100.
But for some weird reason, super kage scrolls only drops at 100(really low chance) and 120, thats it. unlike previous levels, 110-119 never ever drops a super kage scroll.