Relative Rotation Mechanics question (tp vs /rotate)

Hello!
I've been messing around with rotate and there seems to be two different ways of doing it. When you /execute at [entity] run tp [entity] ~ ~ ~ ~90 0, it will move it 90 degrees to the right exactly. However, its rotation value in /data get stacks forever rather than resetting at 360. After doing it 10 times (assuming you begin at [0.0f, 0.0f]), you'll be at [900.0f, 0.0f]. Conversely, /rotate doesn't seem to do this. If you /rotate [entity] ~90 0, it instead moves to [144.60f, 0f]. 
I would like to know when one is meant to be used over the other, and if there is a way to replicate the exact rotation mechanic in /tp with /rotate. Thanks for the help 🙂