iPhone 5S Steps and Map My Walk

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Does anyone use the iPhone 5S step tracker/calorie adjustment? I do, I find it a great motivator to get up and move around more during the day. I then use MapMyWalk (MMW) to log my intentional walks. When MMW logs a walk with MFP, I notice that MFP adjusts my iPhone step calories downward, I assume to prevent double-counting. In the short time I've been doing this though, it seems a little inconsistent when it comes to adjustment amounts. Does anyone else have experience with this, and am I doing something wrong? Is it actually double-counting my calorie burn?