I really do think it is a pricing error (same price to add to a DNS equipped GT/SE), and the R price is correct for GTI/GTD/R. If the GTI/GTD orders get accepted by the factory before they realise their mistake (assuming it is a mistake) then it's their loss, not yours.
Just maybe, it is a genuine price and VW are using it to dampen enthusiasm for R while they can't make them quick enough? That assumes some would pick a GTI/GTD over an R for cheaper options rather than performance.