l6470 Speed Register

According to Datasheet Reference:

In order to convert the SPEED value in step/s, the following formula can be used:

[step/s] = (SPEED X 2 –28)/ tick

That would mean that if I would like to work with step/seconds I have to multiply it like this (stepsPerSec * 67.10886) to get the real Value.

So if I run my Motor with 10 steps/sec for 60 Seconds at 128 Microsteps, the Motor have to make 76800 microsteps.

But if I get (read) a Position Register after 60 Seconds I am always getting bigger Value.

After try and error, I settled down to multiply my step/seconds with 65.9200 to get a 76800 steps.

Could it be that formula in Datasheet is wrong?