BBT vs OvaCue Ovulation differ

I am on 50 mg Clomid days 5-9 and using both methods BBT & OvaCue to test for and confirm ovulation. Every month, my BBT coverline shows that I ovulated on D17 but the OvaCue indicated that I ovulated around days 12-14. Does this happen to anyone else? I am trying to figure out which is more accurate. Please advise. 

Thanks,

SJ