Ok, well, it seems that no one here has ever seen this before! Surprising really...

I gathered some stats, and it seems that on our 16 processor box, all the values are 16 times bigger than they should be for idle and CPU busy time compared with the perfmon counters looking at the same thing.

I dont know if this is an intended behaviour of the stored proc, but it seems you can get close to accurate results by dividing the values by the number of processors you have on your server.

Hope this helps someone!