In the time the slow miner was working, getting nothing (yet), the fast miner already got work in, and got it accepted.
How is that a problem? The slow miner hasn't found a share yet. Whenever he does find one, he'll report it to the pool and get credited.
Because the block changed. We he finds one, its a whole new block by then. All his work on that last block will never be given credit.
Meanwhile, fast miner got 9 on the last block (or 8, or 10, whatever you like). Even if slow miner finds a block this time, fast miner probably got 8-12 shares again.
Over time, the block changing hurts the slow miner more.