Of the list i saw, SEAT was the worst manufacturer for getting less MPG than quoted..
I think all manufacturers are guilty of this - some just get found out more so than others!
In my FL FR+ 170 I recently got 65.4 mpg average on a journey which is only 0.3 off the estimated extra-urban figure. (Most of the journey is motorway based!)
In my Boc I got 54.5 (average) mpg on a journey which is 1.2 more than the estimated extra-urban figure! (Same route, again mostly motorway based!)
Both were runs from Scotland down to the midlands cruising gently down the M6 - be under no illusions - these were eco-runs that I did purely to see if these targets can be met.
Unfortunately you have to cruise at roughly 55-60mph and be quite patient! Don't expect to be able to sit at 70 the with cruise control on and hit those figures
Also worthy of note is both cars had easily covered 10,000 miles by that point. For the diesel that was probably barely even worn in!