I tend not to trust these companies. I recall a time when the (over)estimates were building up to the extent that I had already paid the last bill in the previous bill's payment.. So I send in my own reading expecting a zero bill. They recalculated, and despite having had my money in advance all the time, managed to make out that due to price changes and me ought to have been charged at a higher rate for some of this, I still had a bill, much the same size, to pay. Talk about twisters.
Ultimately they seem to be a law unto themselves.
To be honest I'm unsure I'd want to try to comment on your original question without a bunch of figures to go with it. I don't know what they under or over estimated and what the real reading were. But to be honest, IF you think the bill is in the right ballpark then there's a lot less stress in just paying it and forgetting it.