why did my ovulation day change 3 days before?

hi all- ok so .... I got this App towards the very beginning of the month and when I got AF I marked it accordingly. When AF was done, I made sure to stretch to the correct date. I saw that my highest chance for pregnancy would be the 15th. 
​3 days before the 15th I went to go log in my daily things and now it has changed it to the 12th......
has anyone else experienced this on here?