New to PH Hash rate jumps all over the place

Is this normal? I am running 4 Terminator A2 units, my average hash rate on other sites is 340.
It just seems strange that the spikes are so high and so low. I am only using one p/w argument a=Scrypt



http://tinypic.com/view.php?pic=6olwjk&s=9#.WkexmzT_q3A

It does not find the hashrate, it finds how many shares you have submitted over time, and does math to compute the hashrate.

The more or less lucky you are, the more or less shares found will impact the computed hashrate for a given timeframe.