Yes indeed, I too was expecting a 1 to 1 ratio of disbursement and was also disappointed. I received about 85% as well. However, according to Bitfinex's statement, they received a certain amount from the BCH source and divided it up equally amongst their BTC holders. So either they were shorted by the source or they charged their users a disbursement fee and that is how I see it.
RE: Bitfinex Bitcoin Cash BCH Distribution Update August 02, 2017