<p><a href="https://meta.stackexchange.com/questions/53585/how-do-favorite-questions-work/53580#53580">How do favorite questions work?</a> (emphasis mine):</p>

<blockquote>
  <ul>
  <li>The favorites tab on your profile has a counter indicating how many of your favorite questions have changed since you last viewed that tab. When you click on the tab, recently changed questions will be highlighted. <strong>"Changed" includes comments on or edits to the question and new or edited answers</strong>. This is a way to get updates on questions that you yourself didn't ask.</li>
  </ul>
</blockquote>

<p>The thing is we can't quickly know exactly what has changed; so there are some proposals going on. The following Meta SE post has more about it: <a href="https://meta.stackexchange.com/questions/83292/detail-recent-favorite-changes">Detail Recent Favorite changes</a>.</p>
