wcs1800 current sensor (with sensitivity of 66mV/A) output voltage varies unexpectedly with a range of 10 - 20mA
You are expecting too much dynamic range from a crude Hall sensor like that. Thermal drift alone is 1mV/K which means 7 degrees C change will change the output equivalent to a 100mA current, so the variation you are seeing is equivalent to only a degree C or two. 

Maybe you can use another sensor that covers a lower range (either more sensitive or wind more turns through the core). It will saturate at higher currents.