"all the dates of the Eighteenth Dynasty, are open to dispute because of uncertainty about the circumstances surrounding the recording of a Heliacal Rise of Sothis in the reign of Amenhotep I. A papyrus from Amenhotep I's reign records this astronomical observation which, theoretically, could be used to perfectly correlate the Egyptian chronology with the modern calendar; however, to do this the latitude where the observation was taken must also be known. This document has no note of the place of observation, but it can safely be assumed that it was taken in either a Delta city such as Memphis or Heliopolis, or in Thebes. These two latitudes give dates twenty years apart, the High and Low chronologies, respectively."So that's why they're so contradictory!
Wikipedia article on Thutmose III (probably the Exodus one)
Very irritating. I like things to be cut and dried.