There are many "Dark" Pools around
For example, the user "Vladimer" on this forum owns 400GH/sec of power.
Alone this would make him in the top 10 pools, and he would get a goof share of any generated blocks.
I think the OP means "Are pool admins trustworthy? I suspect there may be some pool owners who are not reporting all the block solves of their pool, and are keeping the unreported profit for themselves."
Most pools, including ours, post all of the blocks found...
If pools were to keep blocks for themselves then the average on their site would increase.
People with big pools might get away with it, but for the small pools it would be too obvious.