From my understanding they seem to do it as they changed from the 1.4 petrol to the 1.5 maybe as a sweetener? most people who ordered a 1.4 didn't know they would have the 1.5 until it turned up, although can't say about deisel's if this was the reason.
We all know that Audi seem to give you absolutely nothing even on a SLine unless you stick your hand in your pocket so doesn't make sense that they started throwing that option in.