You either have OCed the card OR the hashrate is buggy. My 5850 doesnt go over 295KH at all with settings above OR any settings at all.
Did a lot of tests on this the last 2 days. Been going through litecoinpool without a hitch. seems to average out at 350kh after 2 hours dropping to 345 and peaking at 355. Been hearing that the 5870 is getting almost 400kh. So this seems about right. What brand of card/os/system are you running?