1) It is a common abbreviation. One reason could be that F was "Fox" in the "RAF phonetic alphabet" between 1943–1956 and in the "U.S. phonetic alphabet" between 1941–1956. (see reference further on) 2) "The NATO phonetic alphabet, more formally the international radiotelephony spelling alphabet, is the most widely used spelling alphabet. Though often called "phonetic alphabets", spelling alphabets have no connection to phonetic transcription systems like the International Phonetic Alphabet. Instead, the NATO alphabet assigns code words to the letters of the English alphabet acrophonically (Alfa for A, Bravo for B, etc.) so that critical combinations of letters (and numbers) can be pronounced and understood by those who transmit and receive voice messages by radio or telephone regardless of their native language, especially when the safety of navigation or persons is essential. The paramount reason is to ensure intelligibility of voice signals over radio links." "Variants - Aviation: . "Delta" is replaced by "Data", "Dixie" or "David" at airports that have a majority of Delta Air Lines flights, such as Hartsfield-Jackson Atlanta International Airport in order to avoid confusion because "Delta" is also Delta's callsign. . "Foxtrot" is commonly abbreviated to "Fox" at North American airports and some European ones. . "Lima" is replaced by "London" in Indonesia because "lima" means "5" in Bahasa Indonesia. Thus, confusion could occur if a string of mixed numerals and letters was being given." Source and further information: http://en.wikipedia.org/wiki/NATO_phonetic_alphabet

I bet in some situations, the single syllable works better than the double. My little guess.