Bizarre !
Even more bizarre is the fact that the 5 door GTD DSG pushes out more CO2 than the 3 door (and sits in a higher tax bracket)! Yet in the GTI they are exactly the same. I can not fathom it 
Lovely pics of the three door GTD with Santiago's. If they weren't so expensive I would have specced them.
I don't think that is true, the 3 and 5 dr both give out 109g/km
3 and 5 door manual give out 109g CO2 on a GTD (both £20 tax disc), 3 door DSG GTD gives 119g CO2 (£30 tax disc), 5 door DSG gives 122g CO2 (£105 tax disc).
I do wonder whether the gearing on a 5 door DSG GTD is a smidge lower than 3 door because the 30kg extra weight of 5 doors and the 17kg of DSG would've compromised the performance by 0.1s vs the manual or something daft like that. The 3 door DSG GTD is 4.5% more fuel efficient than the 5 door DSG GTD, that can't all be down to the 30kg of additional weight a 5 door variant has over a 3 door variant.