There would have been a reason that they couldn't take a reading of your meter on the day.
- Couldn't access your yard;
- Bins in the way;
- Gate closed;
- Raining;
- Meter reader couldn't be bothered.
Take your pick.
On your next reading of the meter they will get the accurate reading from the period before the estimate up to your next reading. If your previous reading was 9900, your estimate 9975 and your actual use at the end of the next billing cycle 9980 you will only be charged for the 5 additional over your estimate.
I would guess they are a little generous on their estimates but provided it is actually a valid reason they couldn't take a reading I expect they are just protecting themselves. Perhaps you could ring up and enquire why an actual reading wasn't taken and if there is no valid reason ask them to come back and do an actual reading and reissue an appropriate bill.