No chronology of Ancient Egypt can be exact for several reasons.  First, many of the writers of ancient documents would neglect to mention kings or dynasties that they considered unacceptable.  Second, the ancient Egyptians followed a 365-day calendar, rather than the 365-1/4-day calendar that we currently utilize.  These calendars were often astronomical calendars (rather than a solar calendar like we use) based on Sirius, the "dog star."