does how fast your hashrate is shown on cmd.exe matter? yiimpmasterhash refreshes really fast while bsod.pw refreshes slowly
The miner executable describes the performance of its hashes regardless of valid shares. Since the miner only submits valid shares to the pool, the calculation on the pool is different. What you see on yiimp is the actual average of valid shares that have been submitted.
This is why sometimes yiimp will occasionally show a hash rate that is either much lower or higher than reported in the miner.
There's more details on this if you search for "hash rate difference" around the forums here or in Google.