Etymologically there are 3 and 4 way couplings... latin copula just means 'joined', nothing about 2.
But when you are in the context of quantity, "a couple" is a quicker way of saying "between 2 and 3/4/5" .
There are no "rules" in English, only conventions. It is a product of whoever speaks it and whatever they bring with them, and as such is forever changing and evolving without any control or authority.
This whole thread is baffling to me because in my entire life, I have only ever heard people use "couple" to mean "two".