Irregular menstrual cycles are very common and happen for a lot of different reasons. Also, there are different kinds of “regular” periods. Someone could be said to have regular periods even if they happen on shorter or longer cycles than the typical 28-30 days provided that it’s a consistent pattern without significant deviation for that person.
You do not have a good understanding of menstruation and gynecology if you think that’s always the case. There are so many variations of irregular menstruation that trying to exclude data based on irregularities would be very difficult or get rid of a lot of legitimate irregular data.