What do you mean that no one can hear the difference? If you have any musical training you should know to listen for "beats" which happens because as the sounds come in and out of sync. The closer together the notes are to the proper interval, the easier it is to hear those beats on a pair of sustained notes as they come in and out of sync. Even though the tones are so close together than nobody could hear the difference if they were played separately.
The beats aren't as strong as when two slight variations on the same note are played. But they are still easy to hear in that link.