The value of the updated element is the date indicating when the entry or feed was last modified in a way the publisher considers significant. Therefore, not all modifications made to an entry or feed necessarily result in a changed updated value.
For example, consider when a user publishes an entry, and then later goes back to associate a few additional categories or tags with the entry. In this case it is probably not important for the system to indicate that the entry has changed since the content of the entry remains the same. But suppose the user accidentally linked to the wrong site within the content of their entry. In this case the system may wish to indicate to applications reading the feed that the entry is now fundamentally different than the original.