There’s no need to suspect anything. Look at the statement PDF and it’ll tell you. You need to look at where the readings are listed. Most likely the last one will have (estimated) written next to it.
Estimated usage can vary between wildly under and wildly over actual usage. If the typical annual usage for the property isn’t defined correctly by the energy supplier, and this depends on many factors including typical usage over years prior to you even living there, then the estimates will be way off. It’s a guess, basically.
The only way your energy supplier knows this is if you tell them via a meter reading. They’re not psychic and don’t know the property has been empty. Even if the estimated annual usage was correct and resulted in perfect estimated bills, the process would still go wrong in a month where usage isn’t representative of a typical month.
It’ll be “corrected” in so much as the next bill will start on the estimated reading that was too high, and end on the actual reading that you provide next month, and so the bill will be artificially low. You’ve paid a high estimated bill in December and will pay a correspondingly low bill in January. But don’t expect to see anything labelled a “correction” or similar on your bill, because technically there is nothing to correct. Rest assured you’ll pay only for your metered usage. It’s only your month-by-month bills that aren’t representative of actual usage any more - overall it works out right.