I set the start time according to the softstarter but this motor still ramps up much faster, Why?

If you ramped up a motor with no load it will be at full speed very quickly on the softstart compared to when you start it at its maximum load.

The softstart designer design it so the power increases to the set time on the motor.

In reality though it is much different because the motor will ramp up in its own time dependant on machine dynamics such as load conditions.

That is why!