The number next to the tab indicates how many posts have new answers since you last visited the tab.
Here's how I confirmed: I favorited this question when it was asked. Popular Demand's and Kiamlaluno's edits did not add a number next to the tab (see comments below). However, Chris' new answer earlier today increased the count, which confirms this. Here's a screenshot:
Although the activity shows Kiamlaluno's edit, the counter increase is due to Chris' answer.
The highlighting means that the particular question has been changed since you last visited that tab. However, the number next to the tab only counts those questions that have not been modified by you (as the last editor).
In your case, the first question was last edited by you. The algorithm probably assumes that you know that you edited your favorites. It makes sense too... you don't want the bulb to light up for a typo that you fixed. However, when you visit that tab, it shows all questions that have been modified.
Here are the steps you can take to reproduce this and confirm my theory:
- Visit the 'favorites' tab to clear all highlighting.
- Close the tab and go to a favorited question and edit it.
- Now visit your user page. You will not see a
 next to the tab.
- If you visit the tab, the question you edited should be highlighted.