The problem in Unity is that Time.deltaTime is bound to the frame generation time, not to the screen’s frame rate. This was good enough up to Unity 4, but since Unity 5 and the new graphic APIs this is not good anymore. Other engines have solved it, but Unity has been left behind.
Long but detailed and extremely interesting article explaining where the issue comes from:
Sumarized and explained:
Time.deltaTime Not Constant: VSync CameraFollow and Jitter page-4#post-4173886