How is it possible that the Maps App can determine my position even if I use a VPN?

I have enabled geo-localization on my Macbook Pro early 2011 and I am currently using a VPN service. How is it possible that my Macbook can determine my exact position? If I use other websites or Google Maps they show my VPN's IP position and not the real one. Can someone explain how Apple's geo-localization feature works?