The bill's early arrival and the fact that it has been estimated I think is very significant.
With EON price rises of around 9% due to be implemented this month (on 18th January?) it seems to me that EON, in their wisdom, are at least trying to pull a fast one or they could even be committing a deliberate act of fraud or deception.
My reasoning is this:
1) Many customers will probably be delighted to find that their bills are lower than they anticipated. Some people may not even realise it is an estimated bill and will take it as accurate and pay it.
2) With many people finding they are short of money after Christmas, they could realise the bill is lower than it should be, and pay it now thinking that they will be saving money.
3) Those that pay it could be in for a major shock when they receive their next bill because the difference in energy units used will be corrected - presumably by their meters being read between now and the raising of the next bill ... but THE DIFFERENCE WILL. PRESUMABLY, BE CHARGED AT THE NEW, INCREASED RATES, when they certainly shouldn't be.
If you are a subscriber to EON .. then you should take the precaution of taking a meter reading yourself - or of you are unable to reach your meter - call EON and insist that they send a meter reader out to do this for you ...and then ... insist on a revised bill.