I am reading about New Kingdom Egypt. Many events are doubly-dated. For example, Akhenaten's reign is given as 1353–1336 BCE or 1351–1334 BCE. These dates always seem to be two years apart.
I understand the difficulties which arise with dating ancient events, and that astronomical events and other synchronisms are used, imperfectly, to date events.
What specific dating systems are being used to give dates for this era and why are they two years apart?