Ovulation day. Rupture on the 10th day?

My natural cycle duration is 31-32 days. But from past few months I have had irregular periods. After doing a sanography it was found that I had a cyst. I was given contraceptives for one month and in the next cycle the cyst was gone. After that doctor prescribed Ovacare LZ from D2 of period for 5 days and follicular monitoring is done on the 9 th day. I have also been given dydrofem 2 times a day for 15 days after ovulation as I have a history of miscarriage. I have been taking this treatment from past 3 months. My follicle size is around 21-22 mm on the 9 th day. This time the follicle ruptured on the 10th day. I don't understand if my natural cycle is of 31 days then ovulation should happen around the 17th day. Then how do the follicles rupture on the 10th day?