Disappearing Posts

I have noticed that some of my post have just disappeared from some forums. For example, in the AIDS forum a lady posted about her mother having cancer. I responded by telling her that she was in the AIDS forum and she might want to be in the cancer one. Then, I posted a non-relevent post in testimonials and I realized it was in the wrong area and apologized but then it disappeared which I would assume Webmaster did this only...there are others who are told they are in the wrong forum (testimonials) and not removed...just wondering if there is a really huge reason for this?
Thanks, Libby Tweet