Fertility Friend will continue to make analyses and predictions as you enter more data as your cycle progresses. If the ovulation date changes, it is usually because something about the new data you entered made another date look more likely than the date originally proposed.
It can be frustrating to see the interpretation change, but on the whole, this allows for greater accuracy since a few new data points can change the meaning of your chart considerably. The coverline changing is not significant since the coverline is just a visual tool with no physiological meaning. The coverline placement is determined according to the pattern of the chart.