Looks like the pool doesn't take into account partially solved shares. This leads to some inconsistencies (and unfairness), specially when total shares are low, but it's increasingly noticeable as the cluster grows in performance, because there are more chances of not completing a share before the cluster finds a block.

In my case, the cluster has been steadily at around 14-15000mh/s, and I usually get around 0.40BTC per block. On blocks found in a short time this varies a lot: from 0.25BTC to 0.80BTC if I completed a share just after or before, respectively, than most of the miners in the cluster.

I don't know if this is solvable somehow, I use a GPU and the variation tends to even out, but CPU miners are clearly at disadvantage and eventually GPU miners will be affected if the cluster grows large enough.