USB datalogger ground read isn't zero volts
Inputs on these are typically covering a +/-10V range and as you may be using a 12 bit input ADC, the resolution will be 20 volts/4096 = 4.88mV.

Given that this is resolution only, accuracy could be double this error quite easily.

In fact on further reading of the data sheet I suspect that the input resolution (single-ended) is only 11 bits. See this - click on data sheet link then click on detailed data sheet link.