Missed periods and negative hpt.

I've noticed that a lot of ladies on this site have frequent missed periods by a few or many days, but continue to not be pregnant. Irregular periods typically come from longer or shorter follicular phases, with the luteal remaining constant. So my question is if AF being late is from not knowing the true day of ovulation, vs varying luteal phase days, which I thought was uncommon. Anyone have any thoughts on this? It'd be especially helpful if this is you, and a fertility specialist has explained it.