VW ADVISE that the cambelt is changed at 4 years OR 60k.. whichever comes first..
Its down to you as the owner to decide if you want it doing or not. If you choose not, then the dealer should make a note of it on their system, just for future ref.
also to do with that voucher.. it may only be valid at that dealer.. sometimes with mailers, dealers are able to buy into offers VW bring out.. maybe that was one, and this other dealer wasnt part of it. Also you may find a difference in labour costs. Id get you a quote, but it would be cheaper and poss down to that reason.