Made Me Wonder

I was really wondering what's the Google search engine is doing. 2 Hours ago, I checked Google indexes on my Update Blog by doing the site search. And I found out that there are more or less 40 pages that have been indexed already.

However by this time, the number of pages that have been indexed was backed to 19 pages. Lots of old posts were stripped off from the index while the new posts were added on it.

Now, what made me wonder is what really is the requirements of Google so that a post will retain on the index. Does it needs to have lots of backlinks to that posts? Is there any limit of the number of words that a post should have?

But I am also thinking that Google maybe is considering those pages that were stripped off from the index as duplicate content. If this is the real reason, then I think I need to strip some content of the theme I am using for the blog. Because those content, for example, in the sidebar, could be the reason so that Google consider a post especially those with only few words the duplicate of the other posts that I made.