Extruder Stepper dos not stp constant

Hello,

I wanted to calibrate a Prusai3 from Maker farm with a j-Head Hotend.
And there i come to a problem.
I found out that the Extrusionrate need a sertain amount of time to level itself to a constant value.

I used this guide
[zennmaster.com]
after i did this i checked the outcome and there was still an huge offset.

So i make a measurement series where i extrude 5 times 50mm and measured the length. vor one cycle i neded ca. the same time +/-10 seconds
what i got was that i started with 46mm extruded length and ended up with 37,5mm
I mad 2 of this series and its reproducible.

Did anyone have seen such a behavior and has an explanation and Solution to this?

Here are my data

sek extrud distance [mm]
0	46
30	43,8
60	41,1
90	39,8
120	38,7
150	37,6
180	37,6
210	37,5