Quadratt -5v to 5v calibration?

Hi there, I’m checking the voltages out of my quadratt with the -5v to 5v toggle and noticed the negative portion is just short of -5v (something like 4.8v). Is this normal? Is there a way to calibrate?

Usually it is not too big of a deal but I am using the -5v as an offset into a unity mixer.

This will also affect inversion. so 10v * A fully CCW willingly return -9.781v.