Data changing after days pass

Is anyone else experiencing the projected fertile days changing after they already passed. It told me my fir tile days were the 12-18 with highest chance on Wednesday and today it updated and said my fertile days were the 10-16 with highest chance on Monday. It also moved my period up 2 days.  Also it is saying i started to become fertile 2 days after my period. This is the second time this has happened. Why?