My (new) 2016 GTD - worst possible combination for economy - 19" Santiago's and a DSG Box.
My computer checks out to be approx 1mpg optimistic (quite accurate), and where indicated before at over 60mph (or near too it), when I've done a fill up (brim to brim) then manual calculation it's been very close to the on onboard computer.
As Fred says not impossible but you have to be driving carefully to get that. On this one I was sat at just over 50mph for most of my motorway journey home due to roadworks and average speed cameras.

Anyway, back to Fred's question, the figures are mystery. Especially as the new 7.5 GTD has the new 7 speed DSG box rather than the old 6 speed, and if that 7th gear is more of an overdrive, you would expect the Co2 and MPG figures to be a little better.