Number of index pages in web master is different from site:mydomainname

[Jamalon]

Google says one to discover whether my pages is index in Google is site:domain name of my website:

https://support.google.com/webmasters/answer/34444?hl=enas mention in web page above so basically according to that i can know totally pages indexed for my website right:it shows me when type (site:domain name ) 300 but it says in Google web master that i have 100000so which is the real number of index page 300 or 1000000 as web master says and why i get 300 when using site:domain name even Google mention that it is way to discover index paged

[DirkC]

Both measures are not really reliable to know how many pages are actually in the index.

Googles definition on 'Total Indexed': Shows the total URLs available to appear in search results, along with other URLs Google might discover by other means. This number changes over time as you add and remove pages. The number of indexed URLs is almost always significantly smaller than the number of crawled URLs, because Total indexed excludes URLs identified as duplicatesor non-canonical, as less useful, or that contain a meta noindex tag. (source: https://support.google.com/webmasters/answer/2642366) 
=> basically this number contains all the pages Google has been crawling since the launch of your site, and is quite useless as absolute figure, it's the evolution over time which is more important

site:domain.com will show you pages that are eligible to show in the search results, but is not a really reliable measure either  - it's quite possible that more pages are indexed. On very big sites you will see this number change when you go to page 2,3,...Etc It is a good check to see if your site is indexed or not.

That said, the number of pages in the index is not a very important figure, it's rather the ranking of your site for your main keywords which is important, as it is the main driver of your traffic.