Propagation produces permutation. The idea you explain to someone isn't the idea they hear. And even within a single individual, their understanding of a concept will change as they learn more about it and place it in different contexts. Most differences will be trivial, but at what point does an idea actually change into a variation of itself, or even a completely different idea? It all depends on what the people involved find salient or important about the idea, I suppose. And whether or not they have an authoritative source to refer to, or just have to work from vague memory.