There are quite a few references to it online. This would, I think, refer to the statistical concept of deviation, rather than "normal English."
http://www.tidsskrift.dk/print.jsp?id=95966 : "The official statistics allow for measurement of levels of industrial activity in terms of work-years for 1905 and 1915, and thus for the computation of the 'deviant' rate of change for the industrial surge which took place during this decade. The deviant rate of change has been described by Duncan, Cuzzort and Duncan as the only measure which fully takes into account the level of the indicator at the beginning of the period, thus reducing the potential 'ceiling effect'. The rate is based on the residual from the interannual regression line and is defined as follows: [formula]
"In other words, the rate measures the deviance of the actual value of an indicator at Y2 from its predicted value..."
I think I am going to query this in the translation.
By the way, Armorel is certainly right that the "change" is positive--i.e., that it demonstrates the effect that the researchers are trying to measure.
I don't think "discrepancy" need be negative in connotation, but neither am I sure it's correct.