@solitudeDeleting a message is probably recorded as a second update.
In a file system, and most database software, the time that the file, folder, or table was last changed gets updated every time a user adds, changes, or deletes anything from it. The simplest way to determine the last update time is to use the time from the system; otherwise, you need to code a separate time that has to be updated every time a user makes certain changes, which is difficult.
Technical (Software Dev) Discussion:
Consider the problem that arises if you try to use a separate, more sophisticated timestamp, where posting a new message is an update but deleting a message is not. What about changing a message you already posted? Altering an existing message should be recorded as an update if the change adds content, but not if it deletes content or just fixes a typo. There is no reliable way to decide which of these has been done, so the safest approach is to update the timestamp whenever a message is changed, even if it deletes most of the original post. But if you are going to do that, you might as well count deleting the entire message as an update, too, since that allows you to use a timestamp that the programmer does not need to maintain separately.