Agree dealer allocation has a major part to play. Also consider this. If a dealer has sold all their allocation it could be that they give the best discounts. Its worth thinking about. In other words it might be possible to get an earlier build date but you may not get the discount you would have got at the other dealer (who's run out of allocation).
When I placed my order I distinctly remember my dealer saying ".....hmmmm, allocation, (looking at computer) not a problem....."
There are folk on this forum who have pushed for the 8's and 9%'s off list price and then haggled for extras. It would be interesting to hear if they are the ones with extended build/delivery dates.
One dealer would only offer me 4% discount "Maximum" but suggested delivery would be about 4 months from order placement. I eventually got from another dealer 7.5% discount and got a confirmed build date 17 weeks later on the 19th December as being week 4. By the time delivery arrives it will be about 6 months - more or less average. During my wait I have heard of deliveries in three months and over a year.
I think its also worth thinking about any change of specs you make on the way, i.e. after order placement, and whether that might make a difference, but before confirmed build date (you cannot make changes after this date).
Also during my wait I have heard that certain options can cause extended delivery dates: Certain alloy wheel choice, DSG gearbox, Colour etc
There are many factors involved most, if not all, outside of our control.