Different behaviour/gameplay when changing quality settings

I am new to unity and facing this issue, while developing game for android i notice that game will automatically set quality setting according to hardware of device, so i have a jump script while pressing space a cube will jump, i have change the quality settings to fastest the height on which it jump is less, then i change quality settings to fantastic its getting more heigt as compare to the height which was in fastest, so if i run this on different hardware of android i will get different jump height and gameplay on different devices, how to resolve this, and is this a feature of unity or a bug.

Here is the link where you can download the unity package that i have created for testing
Box open “Bouncing Test Quality” scene file, the cube will bounce when we press space key, change quality settings at run time and you can notice different behavior.

What are you using to time the amount of jump? if your using time.time your physics will change with the ramerate, if your using time.deltatime it will stay fixed