I'm not surprised, I spoke to someone from JLR quite a few years ago about fuel economy on the XF when it was doing the rounds for pre-launch test drives. He said that what most people don't realise is that not only are the tests carried out on a rolling road (not sure if that's still the case) so environmental factors are removed, but they are also carried out on the base specification. That usually means skinny tires and very little equipment, so unless you buy that specification you're going to add weight (maybe 100-200kg) and perhaps a lot of rolling resistance if you go for bigger wheels.
I also found that JLR engines tend to be very "tight" when new and don't loosen off until they're over 35k miles at least - sometimes over 70k. My 2010 XJ got around 10% better between 55k and 60k, but my 2016 one managed that trick at about 37-40k - it would easily do 50+mpg on a long run with four adults and luggage