It shouldn't do that. Idle should be about 750 and if the fan is on it raises the idle about 100 RPM. Trans mode shouldn't change it.
I am putting this is quotes because I found it on another VW forum sight. Me thinks you are mistaken or I am not the only one who has noticed this.
"
With DSG, engine idles at 1200 only when selector is in S or manual mode on first gear. Don’t know why, I would hate to think it’s regularly starting in 2nd, but that’s just what I observed. Maybe the clutch disengages when stopped on D. I noticed the same thing you noticed, but for me it’s actually something that I don’t mind, maybe even useful for economy reasons.
Edit: I should mention that it’s known that when you put the car into race mode, it changes to “S”, and when you restart the car, even if it’s in race mode, the transmission goes back to “D”"
Later in the thread, someone posted this:
"This is normal and by design. When the car is in sport or race mode, and the car is in gear, the engine will hold revs at around 1200 rpm, which is about 400 rpm higher than normal. This is to minimize lag off the line."