It's very simple, demo trading is virtual, orders are not submitted to markets, it means you won't get slipped, all stops will be exact! If you trade direct to markets, then the time it takes to execute trades to the market usually means that the price has passed and no longer available, and that is slippage.
If you're unlucky and your broker is a dealing desk (market maker) you will trade at prices they decide, or the dealing desk will have "last look" and delay your order fill and the price has gone.
It means that demo will never be the same as real trading regardless of broker model.
Demo is good for checking spreads and the general checking of your strategy, nothing more.