The Nike + iPod sports kit is just a pedometer. It will never show you the real distance. In your case the deviation is extreme, I agree.
To get the exact distance of your workout you can use a freeware called TrailRunner (
http://trailrunnerx.com). The app allows you to layout your route on a map like in GoogleEarth and then connect that route to your workout. Both the route and the workout data are then stored in the diary on your mac. So you're independed from nikeplus.
There's also a tutorial online on how that works:
http://www.trailrunnerx.com/en_tutorial
Hope this was useful.
- berbie.