The same topic revision will be used when you save a topic again within a certain time frame (one hour by default, configurable with the configuration setting {ReplaceIfEditedAgainWithin}). This is to prevent unnecessary topic revisions when you do several edit cycles in a row. Note that a new revision is always created if a different person edits the same topic, regardless of the time.

The default skin (PatternSkin) gives you the ability to force a new revision during save.