Range aggregation Mode return wrong result

I history water volume every 10 second and try to get hourly water usage by using Range aggregation mode (Volmax - Volmin). I set the sample size = interval = 1 hour, so the result would be a series of 1 hour record which each one should be Volmax - Volmin in that 1 hour interval. But as it shows in the image result is totally wrong and in some row even 0!!!
I only get correct result when use Range mode in Tag Calculation query in report for one time period.

please Any Help