Can this be right?

[4687ah]

Hi, while my E53 X5 3.0d was in the body shop for some paint (neighbour reversed into and then drove off claiming not to have noticed-luckily mate and myself were outside my house-I heard it, mate saw it=neighbour paid for it) the car failed to start due to flat battery. Garage put a machine on,wife picked it up so details are sketchy, which printed out battery knackered.
Car was booked in as its an AUC. Dealer charged it overnight and then confirmed that it was indeed "unserviceable".
They then said that as it wasn't a BMW battery and as incorrect for the car (it was a Unipart one) and that it wasn't covered by AUC warranty so would be approx £250.
The good lady wife, not one to shy away from an argument, as I know only too well, argued that "were we really expected to check the type of battery fitted when buying a car and if it was such a big issue then they should check it before selling the car."
Man understood her point but said in these cases, you have to speak to supplying dealer.
She now phoned Stratstone Derby, who, as I like to speak as I find, we're again spot on saying that they would sort it out with other dealer if they would do it at the warranty rate, and if not we should take it to Stratstone at Newcastle, who would change it for us.
I was a little bit surprised at the warranty wiggle, but don't really care who pays as long as it gets done.
The battery was then changed at the warranty rate and Derby paid.
I have been told that the new battery comes with a 2yr warranty, which from what batteries I have seen mentioned on here in the past seems low, saying that I am just grateful it's done.
Anyway here comes the bit that's got my attention, I was working away during this and my wife sent me a pic of the mpg display which was showing 30mpg, as It pretty much stays at 27.2/27.4 most of the time during her normal week around the doors,is it really possibly that the incorrect battery being on there was taking so much from the alternator that the mpg was lower than it should be?
I am finding this hard to believe and would welcome any views on it. Unfortunately I don't have any details on what the spec of the. Unipart battery was.
Thanks in advance gents.

[LincsX5]

How often do you reset the MPG trip? Could it just be that disconnecting the battery has reset the consumption. I often reset the first consumption figure to see what I am getting on a certain trip, e.g towing the caravan, and leave the the second one for a a long term overall figure.