Time.deltaTime is the time interval between current frame and the last one,
which is the same than the time interval between two call to the Update() method.
This means that Update() is going to be called a different number of times
depending on the performance of your game.

On the other hand you have the method FixedUpdate()
which works essentially as Update() but with the difference
that it is going to be called a fixed amount of times per second.
Naturally Time.
fixedDeltaTime is the difference between two calls of FixedUpdate()
and it always holds the same value.