A partner of my company (not sure whether i can tell their name here - ill use example.com instead) asked me yesterday, why there is such a big difference when they enter the site command at google at first with www and then without www, to check the number of indexed pages.First of all, the Google Webmaster Tools show about 9.000.000 indexed pages.site:example.com provides me about 10.000.000 hits.site:www.example.com are only about 4.000.000 hits.I first thought that they have some subdomains registered, such as blog.example.com, or test.example.com ...But entering site:example.com -site:www.example.com to detect pages from subdomains other than www, brings up exactly 2 hits.Where are the other 6.000.000 ?The next thing is, providing google a little bit more in the query such as site:www.example.com in (or other linking words) brings me more hits than the blank site command with www (site:www.example.com). Sometimes about 2.000.000 more than the blank www query...Is the number of hits really that inaccurate?
How reliable is the number of hits provided by Google site: command?
google;google search console;google search
All document counts returned by Google are estimates.  They are often very poor estimates.  They are often wildly inaccurate.From How precise is the number of results in a site: query?site: queries attempt to estimate how many pages are in our index, but we would never claim that it is an exact amount that is completely accurate. ...Once you get past a few thousand pages, that is not all that useful as a metric.From Matt Cutts's comment on Is Google Guilty Of Deliberate Query Sabotage?:We try to be very clear that our results estimates are just that--estimates. In theory we could spend cycles on that aspect of our system, but in practice we have a lot of other things to work on, and more accurate results estimates is lower on the list than lots of other things.You also might be interested in reading more about it:Why Google Cant Count Results ProperlyDatacenter comments - Matt Cutts talks about site: estimates at 3:00