That's because you use the default "detach pieces" option of chilliskinner.
http://www.chilliweb.co.uk/chilliski...ter_Truck1.htm
If you follow the truck tutorial, you'll notice that you can achieve better results if you're weld most of the parts together. It gives you a more efficient layout for texturing and makes the re-assembling of the model (by morphing) a lot faster.
You should try to have as few parts as possible.