It's quite possible the dealer didn't stamp the first service if it came with a 'free first service' package, which isn't uncommon on new car deals.
So the 25K one is likely the second service in reality.
Or, as mentioned already, if it was on long life and the car didn't do a lot of short journeys, it's possible the oil quality sensor in the sump didn't deem it necessary to be changed until 25K. As much as it goes against the grain of car enthusiasts and common sense, long life oil is actually designed to be just that
Either way, I wouldn't let it put me off personally if the car is in good shape and a good price. It's just an engine and should be treated as a consumable. The main structure and interior are the important bits.
At 6 years old, it also due a cambelt/water pump, so factor that into the haggling. It's a £549 job at VW.