As already noted there are ...

As already noted there are many factors that may be influencing the installed accuracy of the readings you are getting from the two meters. You do not state how big the discrepancy is - if they are both good quality electromagnetic meters with ideal installation conditions, then a difference of say 2% or less would not be a cause for concern. 10% or more could indicate a serious problem. Don't forget the telemetry system - incorrect scaling, sampling rates on plc's, etc etc can also give rise to differences.