actually the calculation is correct but the way they simply estimate is wrong

for example my bill in Mar, it is still 17m3 but Apr they estimate 6m3...
then May they come real reading 29m3...
by right Apr should be somewhere around 17-18m3 as you can see total from Apr and May is 35m3 but they charge extra on the 9m3 as over 20m3 got higher charge.

same thing happen again on Jun with estimation 12m3 and real reading on Jul with 28m3...
i ask them to get prorate over 2 months like tnb but don't want reply me...