Sitemap & Google

I don't get it, I've always had lots of pages indexed by Google, but recently the number has decreased, I have not made any changes in my forum, these are the stats:

Total URLs Submitted: 67761
Indexed URLs: 30799

But a search on the site in Google only shows: 4870 indexed pages ?

How is this possible, to have a much lower number indexed in Google than the number indexed according to the Webmaster Tools ?

the site: command is known inaccurate.