Deepbit may actually have a slight advantage if a block is solved by both Deepbit and a small pool at almost the same time. There is a fairly large chance that Deepbit will solve the next block as well, and if it does it will probably include the block solved by itself and not the one from the small pool in the chain, because that is the one it knew of first. The fact that Deepbit hardly ever gets invalid blocks may be evidence of this.