"Hashes" value increase when pool accepts completed mining job with some difficulty. Difficulty corresponds to the statistically average number of hashes which is needed to calculate this job. Typical difficulty values are in range 50,000-100,000, so with just single Web miner you may need few hours to get a chance to get some job done. At this moment when job is done, value of hashes will be increased according to the difficulty. If you have many active miners, this process looks linear.
We are considering adding ‘raw hashes’ statistics to be available for users, so they will be able to see how fast they really mine when the hash power is low.