CD 3-7 or 5-9

Why do some doctors choose to start Clomid on CD3-7 and some choose CD5-9? First round of 50mg clomid was CD5-9 (Nov 19-23) and we checked my progesterone yesterday with a level of 0.46. The doctor wanted it >15. She is increasing my dosage to 100mg CD5-9 with a follicle check via ultrasound on CD14 of this next cycle.