The variance in your effective hashrate is caused by the pool calculating your rate based on the number of shares you submit in a given period.
If you're lucky you'll submit an above average number of shares and the pool will show your effective rate as being higher than usual. Conversely, if you're unlucky and submit fewer shares in a period then your effective hashrate will drop below the hashrate that you're actually mining at. Over short periods of time this effect can result in some quite large variations.
If you look at your average hashrate over a 24 hour period (assuming you mine constantly during that time) then it should be close to your actual rate.