Fertility window on different apps

I have been ttc for almost 2 years #3, getting pregnant has always been hard for me. I was 5 years with #2!

I have been using another app until last month when I downloaded this one. I noticed with this cycle that my window for fertility is different. Way different. Today is the only day the two have in common. I was wondering if anyone else has this problem and which was more accurate. I would have thought they would use the same calculator.