Peter [the Deacon, who wrote in the twelfth century] tells us that Constantinus was born at Carthage, by which he probably means Tunis, since Carthage was no longer in existence, but went to Babylon, by which Cairo is presumably designated, since Babylon had ages before been reduced to a dust heap, to improve his education. His birth must have been around 1015.
Lynn Thorndike, A History of Magic and Experimental Science, vol.1, p.744. Thorndike does not give any further reasons for those substitutions, i.e. why anyone would have referred to Cairo as Babylon, if it was common among writers of the time etc. He also translates “regis Babiloniorum” (of the king of the Babylonians) as “of the caliph,” apparently for the same reason.