Greek letter societies

The parade of nations, opening the Olympic Games in Athens, is being conducted in alphabetical order according to the Greek name for each nation. I took a couple of semesters of Greek in college, so I wasn't surprised to see the "Saint" countries at the front of the line, with places named for female saints (e.g. St. Lucia) preceding places named for male saints (e.g. St. Vincent, San Marino), because the Greek word for "Saint" is also the word for "holy": Άγία (hagia) is the feminine nominative singular form of the adjective, Άγιος (hagios) is the masculine equivalent. That little quote mark in front of the alpha -- a rough breathing mark, actually -- is sounded as English "h", and sometimes proceeds a vowel at the beginning of a word, but doesn't affect the alphabetical order. The "Z" countries get to come much earlier than usual -- zeta is the 6th letter of the Greek alphabet, while countries beginning with "Ph" or "F" come near the end, because phi is the 21st letter.

More puzzling was seeing some countries whose names begin in "B" in English in the expected order -- Belgium, Bermuda, Bosnia, Bolivia, Brazil -- while others marched in the midst of the Ms -- Barbados, Bahamas, Belize, Benin, Botswana. I caught a glimpse of one of the country name signs, and it looked like the Greek version of the names of the latter group began with ΜΠ -- "mp" if we wrote it in the Roman alphabet.

A quick Google turned up this list of country names in Greek. Barbados in Greek is Μπαρμπάντος -- Mparmpantos. Botswana is Μποτσουάνα -- Mpotsouana.

The only explanation I can think of for this is that in modern Greek, beta is pronounced closer to the English "v" (Vietnam and Vanuatu got to march close to the front with the rest of the beta nations), so the mu-pi combination may be the only way to represent the wholly foreign sound of an English "b".