I think that it can also happen when someone posts something, and gets a few replies that disagree, but the disagreement is based on what the poster sees as a misunderstanding. I know it's happened to me on both sides. I have really felt like, 'all right, I need to say this again because I don't think we're actually disagreeing, I just didn't get my point across'.
For example, in the thread about kids' bodily functions and Facebook, several posters said that they thought it was a problem because of lack of dignity/grossness, others said it was different when it had to do with kids. I posted that I didn't see why it made a difference that they were kids, and posters replied explaining their perspective on why talking about bodily functions of toddlers didn't have the dignity problems that it might if they were talking about an elderly relative. I actually agreed with this, and realised I should've clarified, saying that I didn't think it made a difference as to whether or not it was gross. The dignity thing, though, I did see their point. So, I can understand the desire to be like 'but...I meant this other thing, which is slightly different than what you replied to!' I usually try NOT to keep going in those cases, because my life and happiness doesn't really depend on having everyone understand exactly what I meant, but I do get the urge. And if it had been a topic more important to me, I might have kept replying.
But the other side of it is that when disagreeing with someone, hearing, 'You just don't understand!' is very frustrating, because sometimes..yeah, I do understand, I just disagree. There are times where I feel that 'not understanding' is valid, though, and it can be hard to tell which is which.