Total indexed web pages less than the sum of sub directory URLs

Hi List,
I noticed a strange issue for my website, let's take it as xxx.xx as an example:

Google has indexed 4000 web pages, as per the output from "site:xxx.xx", but when I use "site:xxx.xx/abcd" for query, it indicates more webpages than 4000

Can anyone explain it please?

 

Please ignore this, I got some info from here:
hxxp://www.searchenginejournal.com/google-site-operator-inaccurate/14519/

 

There is some error in result.please check in other tools at least in 2.And compare both.