Limitations of MOSFET-pair Current Mirrors
One of the reasons could be that since you have an extra load the resistance of the right side is different compared to the left side mirror. So this would cause slight deviation in the mirrored current. Or the mosfet to the right is not in saturation ! Because 100ma is a lot of current and the voltage that it would create at the gate of the left side Mosfet would be high such that Vds of rights side mosfet is small compared to Vgs (Since the same Vgs is given to the right mosfet) therby putting the right mosfet in the triode region. Checck if the mosfets are in saturation first , if they are then the deviation would be purely because of differnces in Vds from the 2 transistors.