Donkey kick scaling less then Shoryuken when used with level 3

I've just recently picked up Ryu and when trying to learn his combos I've noticed that using donkey kick to combo into level 3 always scaled way less then shoryuken making it the better option despite shoryuken doing more damage. Why though? I also noticed that level 3 goes from 5 frames of stratup to 2 when use from heavy shoryuken, only heavy though. Why is shoryuken so weird with it's super cancel?