Manual Mode left drift

I have a rover that gives me some problems in manual mode. when I use the acceleration lever to obtain a straight trajectory, the rover advances with an unwanted left drift. My RC controls the throttle with the left stick and the steering whit the right stick.
I post 2 videos that show this behaviour

VIDEO 1 - ( )
VIDEO 2 - ( )

Anyone knows how can I solve this problem?

You are driving in manual mode, so you see what the mechanical setup (friction, weight, distribution, manufacturing differences in the hardware you are using), rc trims (should be zero) etc. are doing to your vehicle.
You only drove forward in your videos and the left track is running slower than the right track. If you drive backwards and the left track is running faster than the right one, it could be an issue with the neutral signal.
Try to adjust the servo_output center value and see if that helps.

Just as @count74 has said, thats perfectly normal when driving in manual mode. the ESC for both motors are calibrated in different values, when you send 2000ms signal to both of them, the speed of both engines are not the same.
You can try adjusting the servo_output center value to improve the situation. Also, forget about introducing any mix in the transmitter, ArduRover should be doing all the operations.

Thanks for reply
I tried to change the servo output parameters from the INITIAL SETUP/ MANDATORY HARDWARE/ SERVO OUTPUT and goes a bit better but the problem remains.

The starting configuration of the parameters for the skid steering was:
#1 throttleleft min=1100 trim= 1500 max=1900
#2 throttleright min=1100 trim= 1500 max=1900

today I tried to change to (after some test)
#1 throttleleft min=1100 trim= 1500 max=1900
#2 throttleright min=1119 trim= 1519 max=1919

the results is that at low velocity the rover has a little right drift, while at high speed the rover has the same left drift.

So can you explain me how the three parameters are related to the stick position?
I tried to test only one parameter per time but I can’t understand the relationship with the stick position.