In "how method came to be" I think that oogabooga has the best answer for you when he said that method was just a word chosen to differentiate it from functions, which was first used in Smalltalk.
You're actually right, though. If it looks like a function it is a function, probably. But in C++, which has OOP and heavily encourages its use, you need to clearly distinguish the parts of the class from everything else in the language. So all methods are functions, but not all functions are methods. Ultimately, I think you're searching for whether such a distinction is important and I think it is. You're more likely to express a program in terms of classes and objects with C++, and you don't want to be talking about one feature and have people plan to implement the other feature because of an ambiguous name. A method works with a specific instance of one class (whether you pass it in or not) while a function doesn't have to do that in order to be a function. In fact a function could be part of a design on a higher level and have to work with several objects.