It's fairly simple really, I use my car as my daily driver and my main commute of 40 miles is spent at 40-60mph in constant traffic, so perfect conditions for good MPG but no choice unless I act like an idiot and try to pass everyone.
This usually gives high 30s to 40 as seems to be the norm for this average speed judging by the comments from the VWROC forum linked earlier. Was never that keen in rushing to work anyway 
At the weekend when enjoying the car it's usually low to mid 20s so this averages out at what you see.
That's what I love about these cars, best of both worlds.
Thanks for the clarification and that's exactly what I meant. The reason for this - unusually high - 31.9 mpg is then down to the fact that you are in a position of doing 1000miles/month in constant flow, between 40 to 60mph. No wonder you're getting these numbers as that's the ideal scenario for high mpg, in any car. For everybody else using the car in shorter or more spirited trips on average (as for example: myself, Booth11, and the OP) he'll never get to see these numbers long-term. The weekend drives which are closer to the type of use I associate with these cars are around 22 for you as well which is in line with everybody else's figures so at least the OP can relax knowing that nobody's getting better economy out of their car - it's only a matter of usage.
That's why I always say economy comparisons without detailed description of the way the miles were driven are totally pointless. The difference of modern, advanced engines such as this one compared to 10-20 years ago means that at least in low load conditions they are able to drink a lot less than in the past, but that's just about it. The moment you start to spool the turbo everything goes naturally out the window.
PS. On the other hand, If 90%+ of your miles are 40-60mph steady, you don't really see any benefit (performance wise) in driving a car like an S3/R/etc. An A3 would have provided exactly the same experience (performance requirements in that scenario are practically non-existent) and guess what, with probably 50-60% the fuel you're using now?