broken threads

Using KMail 1.4 in SuSE8/KDE3.0.0 This happens fairly regularly to me. I will be reading through the messages, find someone has asked a question I can answer and reply with the solution. I continue scanning the messages and find that farther down the list, someone has already answered the question I just replied to, but their reply is no longer part of the original thread. What causes some people's replies to become disconnected from the thread? Is this something going wrong at my end or just some wonky way the mail tool that some people have picked decides to handle the mail headers? C.