This has been happening for the last h.
Remember that share finding is again exactly the same as block finding and the full network block finding.
However, with shares there are a lot more of them.
A 'miner' (or proxy) should be finding about 18 shares per minute - or 3.333s per share.
Thus in a day that's around 25920 shares.
Looking at the CDF table, you'd expect a 900% share, on average, once every 8103.1 shares, so on average about 3.2 a day.
A 900% share is 30 seconds - and that will affect the hash rate estimate - the hash rate is just an estimate based on shares submitted.
So you expect your hash rate to vary quite a bit - look at the shift graph to get an idea about how much variance you're getting.
... and aside: if on some other pool you don't see that variance, then they're faking the results