Im so confused ??

Ok so my fertility doctor says ovulation days are from your 12th -15th day from the begining of your cycle so then why on every app according to my phone its differnt the length longer I've bin checking and having light lines on the wrong days wasnt really trying until now had borderline pcos and i beat it lost weight and changed my diet and lifestyle thank goodness finally have a period of my own but now im lost as to when i actually ovulate ugh wich test is best to check???