Indexed URLs

When I submited first time sitemap I had about 4000 urls and after one week google indexed about 2000 urls. Now after 6-7 weeks I have 9000 urls and I still have about 2000 indexed urls. I was fallow this number and find out that it is changing every 2 days . After one week when it reach 2k it just stoped. I was getting maybe 10-15 urls every 2 days and yesterday when I looked that number foll even under 2000.
But when I tipe site:mysite.com in google I have 9k indexed pages.

I don't understand why does it happening? Can anyone explain it to me?

Anyone?

From Rand Fishkin's SEO Guide:

Fluctuation in Search Engine Page & Link Count Numbers The numbers reported in "site:" and "link:" queries are rarely precise, and thus we strongly recommend not getting too worried about fluctuations showing massive increases or decreases unless they are accompanied by traffic drops. For example, on any given day, Yahoo! reports between 800,000 and 2 million links to the SEOmoz.org domain. Obviously, we don't gain or lose hundreds of thousands of links each day, but the variability of Yahoo!'s indices means that these numbers reports provide little guidance about our actual link growth or shrinkage.