If a user clicks Preview or Show Changes before hitting Save, then it won't send a notification.
Anomie explains why, in the penultimate comment here:
"Looking at the code, it appears that Echo detects an "undo" by looking for the field wpUndidRevision in the submitted form data. This field is present in the form created when using the 'undo' link, but it isn't maintained if you use the Preview or Show changes buttons on the edit form. So "undo → Save page" would be notified as an undo, while "undo → Preview → Save page" or "undo → Show changes → Save page" wouldn't be. Anomie⚔ 10:43, 18 July 2013 (UTC)"
a second example is mentioned here: