The "model year" and the "build date" do not always coincide.
Wikipedia explains the concept in detail -
Model year
Volkswagen defines their "model year" in the following way:
MY11 (B) covers mid-year 2010 to mid-year 2011
MY12 (C) covers mid-year 2011 to mid-year 2012
MY13 (D) covers mid-year 2012 to mid-year 2013 (or until whenever production makes way for the Mk7 Golf)
etc.
As already mentioned, the 10th digit of the VIN represents the vehicle's "model year".
If it's a "D", then it's definitely MY13.