The POOL 'Guesses' at your hash rate by looking at the rate you submit valid shares. It can't see the hashrate your miner reports from the software.

The rate you submit shares can vary according to your 'luck' in finding them so the hashrate the pool shows will ALWAYS go up and down.

In addition the pool may reject your shares for various reasons, most of which are to do with your miner config (EG overclocking can cause invalid shares) or internet (EG slow internet means that your shares may be stale), intermittent internet means your miner may be finding shares but unable to submit them - somewhere it says that difficulty is varied so you submit 5 - 15 shares / minute so it is quite possible for your miner to getwork but not be able to submit the share a few seconds later.

(EDIT: apologies - misread the post chain and you HAVE shown your share values...)