My guess is that it gets messed up when you double back on your route (I watched the animation of your runs in Player).
In your "loop from dad's house" run, which is a simple loop, the markers look fine. But your "boxing day" and "feeling sluggish" runs, both of which have have retraced sections, the mileage marker issue shows up.
My guess is that retracing the same path gets the software confused sometimes. I've seen the same thing occur, with other software, in posted bike rally routes where the route was out and back on the same roads.
