Battery calibration failed

Hello, I have bought a NXP-dronekit quadcopter and Tattu 6750mAh 14.8V 25C 4S1P Lipo Battery. I have tried to calibrate the battery, using an ampere meter and a voltmeter, so the voltage divider, and amps per volt parameters are set “right”. Now, I have tied the drone to the ground, with propellers on, a voltmeter on and tested the throttle with an RC controller and visualized the voltage measured from the drone in QGround. When the throttle goes at 50%, I observe a large deviation between the voltmeter measurement and QGround measurement, around 2.0-2.5 V. When the throttle goes at 90-100% the deviation between QGround and voltmeter is 3-4 V. No matter how much I change the BAT1_V_LOAD_DROP parameter (I tested in a range of 0.34-2), I observe a large deviation between the true Battery voltage and measured voltage from QGround. The smallest deviation I observed was at around 1.5V, after I set the BAT1_V_LOAD_DROP at around 2.
Also, I calculated the internal restistance of the LiPo at around 33mOhm, which I am not sure if it is normal.
Any thoughts?

Drone kit:KIT-HGDRONEK66: NXP drone kit | NXP Semiconductors
Battery:Tattu 6750mAh 14.8V 25C 4S1P Lipo Battery with XT90 [TA-25C-6750-4S1P] - 93.00€ : RC Models, Online Model E shop : Modellsport

Thank you!