Are the water meters at the ...

Are the water meters at the reservoir inlet or outlet? If at the inlet, then the cause maybe due to inaccuracies caused by lack of calibration. If at the outlet of the reservoir then check for any loss through overflows or reservoir leakages.  At the same time check for Rising mains to identify any leakages, check other loses eg at AV points. Check if you do frequent flushing of the line. 

The other option you can install another meter in series with the reservoir one.