That's probably typical use. Perfect use is is %99.9.
Most studies show that a user can miss quite a few, such as 4 days in a row[1], and not ovulate.
[1] http://journals.lww.com/greenjournal/Abstract/1992/06000/Eff...
http://www.contraceptionjournal.org/article/S0010-7824(04)00...