The pool determines your hashrate based on the amount of shares sent by your mining rigs (workers). This value could be a different from reported hashrate (in the mining software).
We've noticed that some miner use a special proxy server that filters out low difficulty shares, only submitting shares that solve the block. This will show up as the miner with the low hashrate finding many blocks. We do not know why exactly do the miners use the proxy servers: maybe they want just to reduce their internet traffic.
If we find miner using proxy server we add a special "Proxy Detected" tag on his statistics page.