That is the answer to your question, in realtime the market order fills are based on the live market. The Slippage property is only used for backtesting and not in realtime.
If you don't like the fills which you are getting from market orders and want to have a fill on a specific price you need to use a different type of order where you can use a specific fill price. Market orders will fill based on the current market and likely will not be the MIT price that you specified.

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