You'll completely whiff on any given block just as often as you get one share.. meanwhile the other guy gets 6 in. If he has a little bad luck, he gets 5. You have a little bad luck? you get 0. But, it all evens out right? A little bad luck this time, a little good luck next time. Of course every time you have a little good luck, you don't get 2, you'll still get 1. He'll get 7
Miners aren't paid based on some sort of weighted system where it's the # of shares per block you submit. It's the total number of shares you submit vs the total number of shares all miners have submitted.
Say the slow miner's average hash rate is about 1 share every 2 blocks and the fast miner is 9 shares every 2 blocks. Look at the slow miner over a period of 4 blocks. Maybe he submits 0 shares in blocks 1-3, and 2 shares in block 4. Or 2 shares in block 1 and no shares in block 2-4. Or one share in block 1 and one share in block 4, with no shares in blocks 2-3. The end result is after four blocks he's submitted 2 shares.
The fast miner, on the other hand, would average about 18 shares over these 4 blocks. In total after 4 blocks, on average, we'd see 20 total shares. 2 from the slow miner, 18 from the fast miner. And 2/20 of the coins being generated by the pool would be getting paid to the slow miner. Which specific blocks the slow miner submitted the shares on doesn't matter.
If it's more clear, raise the slow miner's average speed to 1 share per block. Whether he gets 1 share each in blocks 1-4, or 4 shares in block 1 and 0 in blocks 2-4, he still submitted 4 shares and is paid 4/total_shares_in_pool. How the shares were spread between the blocks doesn't change this payout.
(I'm assuming something like proportional or PPLNS payout of course. DGM doesn't use a proportional ratio although steady miners over time end up with the exactly fair proportional income based on their relative speeds.)