Well, sperm can live up to 4 days in prime conditions (the right cervical fluid) so if you ovulated earlier than day 21, it is quite possible if you had egg-white or even creamy cervical fluid. Did you notice if you had this quality of cervical fluid in days 16-21? And are you temping to confirm ovulation at day 21?
I have been temping for 9 months now. January I peaked on day 21, February day 18, March day 19, and April day 21 again. I wasn't really paying too much attention to the cervical fluid, TBH. I should have been.