It doesn't really mean anything, other than her body doesn't follow a schedule exactly. Also, it can depend on how she counts the days. If you go by the calendar date, you have to keep in mind that not all months have the same number of days. A lot of women have slightly irregular cycles; some women's cycles are very irregular.
I doesn't mean anything. Haha. Her cycles just a little off. A lot of the time when girls hang out with other girls a lot, their cycles sync up do everyone's cycle gets a little off for a while. That's why a lot of girl teams would get their periods at the same time.