Hopefully this will help. It's a double whammy...
Because the server is reporting wrong khash and the payments have gone down, we (everyone seeing this issue) are actually losing 25% of what we're doing somewhere.... HUGE deal...
Considering the minimum hashrate over the period you are viewing was 0MH/s, you average number is not going to be correct either. Server isn't reporting the wrong KH/s anywhere I have seen.