"No! I mean..."

I bet most of us have encountered the situation where someone "agrees" with you by saying "No! I mean it supposed to be..." blah blah blah and then repeat what you've just said.

Why did they have to do this? It's really annoying. And when I reply "Well, it's exactly what I've just said", they go ahead and say "Yea, but I mean..." blah blah blah switch to other aspect of the topic. Arghhhh!