the following is from wikipedia
Koine Greek arose as a common dialect within the armies of Alexander the Great. Under the leadership of Macedon who colonized the known world, their newly formed common dialect was spoken from Egypt to the fringes of India
http://en.wikipedia.org/wiki/Koine_GreekKoine Greek is the one that the Egyptians learned and its pronunciation is the pronunciation the Egyptians used
Koine Greek was replaced by Medieval Greek, since the foundation of Constantinople by Constantine I in 330 A.D
at that time Coptic language was already standardized and used to write many Coptic text
maybe Greece changed its pronunciation after that date (330) but the Egyptians did not follow