Basic cars tend to get the smaller version, like 1.4 16V petrol engines, I'd think that all TDI and higher powered engines like the 1.8T would get a version of the full undercover to control air flow and reduce engine noise.
Edit:- try looking under 2006 instead of 2005 and you will find that 6Q0 825 235 E was the last number for that part, as it has been listed as cancelled and no new part listed, maybe search under that number and/or check for any of the listed aftermarket variants listed against that part in that listing.
Another Edit:- all very confusing, more than half the hits/pictures that searching under that number show the short basic cover and the rest the fuller cover!
I think that you need an answer from someone that has had one of these cars from new!
Final Edit:- can you look under your car to see if there are any fixing points there for the fuller cover, from the pictures that you have put in your modified OP you can see where the extra fixing points should be located roughly on your car.