Why does the Page Overlay only show on index pages? What does index page views mean?

I have two questions and I hope I can get some clarity here.

  1. I would like to know why the Page Overlay only show for the index pages? Wouldn’t it be useful also to see the Page Overlay for every page? Why is it only for index-pages?

  2. How should I interpret the pageviews of an index-page?
    For example I have a startpage with 200 page views but its index-page only have 100 page views. Does it mean that the visitors who made 100 page views did not continue to other pages?? And when I look at the Page Overlay report it only shows me the behaviour of the 100 visitors and not the 200.

Hope somebody can explain it to me so I understand. Thanks a lot for a great function in Piwik!
All my best regards!