Saur on line, have their bills available in pdf format …
I opened July 2017 and it definitely says “estimé”
The December one then give the actual reading and instead of “estimé” it says… “est”…
The July bill is estimated at around half the previous annual consumption… which, in your case, seems to make sense if the previous bill (Dec) was twice this current one…
When you/or someone visits… do you read your meter and check the figures?
Have you done a “check” that there are no leaks your side of the meter???