Have you considered that the Bontragers probably have rim strips adding to the weight, while the Ksyriums don't need rim strips?
The LBS said the new '08 SLs needed rim tape. I looked at them and assumed I didn't need rim tape but they said they did (in one area on the wheel. At the same time, they went ahead and put a cassette on for me and tubes/tires. I won't know for sure if there is rim tape on the wheels until I need to change a tube. However, when I changed the rim tape out on the front Bontrager Race X lite last month, I weighed the new roll (per wheel) and they are only 10 grams per wheel (so I doubt that is the source of the extra weight).