It is definitely testing error (or more likely, the normal variance from the precision of the test.) You numbers look like their are either from a strip reader or colorimeter. If they are from a strip readier they are just not to be trusted. If they are from a colorimeter such as the LmMotte Waterlink system they are in spec because, if my memory serves me correctly, the LaMotte UDV TA test has a precision of +10 ppm / - 20 ppm for a variance of 30 ppm. (The Taylor titration test, which does not use any fancy meters is actually ore precise with a precision of +/- 10 ppm!)