Did you add in the actual price per litre changes for that comparison? Don’t forget the R is running on 98RON.
For me on 10k miles a year, I deduced that my GTD 44mpg average (which seems quite normal on the forum) is 66% of the published combined figures, and so on that basis I would expect 26.4mpg from a “40mpg” R, or 31.0mpg from a “47mpg” GTI.
I took fuel prices to be 127p/L for RON 95 (GTI), 130p/L for diesel and 135p/L for RON98 (R).
£1323 for the GTD, £1876 for the GTI and £2321 for the R, doing 10k miles at the above prices.
The petrols do seem to be a little closer to the truth in their actual mpg vs published combined than the diesels are for most of us here. Maybe ss GTDers push our cars harder than the average GTI/R owner needs to (I think this is probably true of me). For that reason, I expect the average mpg for the GTI and R to be at 35mpg and 29mpg respectively (from mpg discussions here, biggest running in improvements reported with the R), making 10k miles fuel costs:-
£1323 for the GTD (unchanged), £1662 for the GTI and £2132 for the R.
For the average among us then, an R is going to cost £67 a month more than a GTD and £39 a month more than a GTI in fuel.