Skid steering not full throttle?

I don’t know if I’m doing something wrong but I’m only get a range from 1500 – 1700 forward and 1500 – 1300 backwards from the pixhawk ch1 and ch3 at full throttle? Is this correct, if I turn left or right the I get the full range of 1100 – 1900. I mean I lose a lot of power forward and backward if this is the case? Or I’m doing something wrong?

Or is this because of the steering function, that you have throttle left so you can steer at full throttle. But in that case is it not better to just reduce throttle on one engine if you steer at full throttle, or?