Node update date changes when saving without changing anything
If you save a node then it has changed according to Drupal...regardless of whether or not any specific property/CCK field values have actually been changed.

You can find the evidence for this in the node_save() function which contains this line of code which runs for every single call to the function:

// The changed timestamp is always updated for bookkeeping purposes (revisions, searching, ...)
$node-&gt;changed = $time;


Drupal does not do any sort of diff against the current and new revisions of a node when saving, so if you've pressed "Save" it makes the assumption that you wanted to add a new revision of the node.