Why so much variance in mining pool reward?

[D_Thomas]

So I finally got my PC hardware working where it is reliably crunching a consistent 400MH/s. The reward on Slush's Fund varies from 0.0100 BTC per round to about 0.005 per round. It seems like it's either close to 0.010 or close to 0.005 but seldom somewhere inbetween like 0.050.

Why is there such a variance in the rewards?

[SaintDevil]

You are rewards based on the work you have done for the block found and it's duration.

[D_Thomas]

You are rewards based on the work you have done for the block found and it's duration.

I understand that but part but if my PC is putting out a consistent volume of work, it would seem like the reward would be more consistent. There are thousands of people in the pool. I would guess the average productivity per miner is realtively stable for a large pool, so I don't get why the reward varies so much between rounds.

Is a chunk of it just luck? For example, my PC gets lucky and finds the solution to several problems quickly during a round versus getting stuck a long time on a single problem?