Hi Patrick
In our company we find that our Sales NPS always sits much higher than our Service NPS. This is partially because in our industry usually a purchase is a positive emotional purchase for the customer, they look forward to making the initial purchase. In your industry is the initial sale a exciting moment for the customer or is it a bit of a grudge-purchase, you know "I don't really want to spend money on new shock absorbers for my car, it isnt all that exciting, but I do have to" compared to "Yay I can't wait to go out and buy some new shoes / x box games / chocolate / mobile phone / ipod" etc
Having a purchase that is a bit of a 'grudge' purchase could explain having a sales score lower than the overall company score. If it isnt that then you really need a little more information to start understanding
Do you have a qualitative question alongside your NPS question in order to get a little more feedback from your cutstomers?