Google doesn't show all of the backlinks to your site from what I've heard. I've done both: link:http://yourdomainhere.com for Google and Yahoo. Yahoo reveals much more. There are also a bunch of other tools on the net and they seem to come up with numbers closer to Yahoo then Google in terms of the total number of backlinks.
I have far more than I thought... 2950... But then when I go to the bottom there are 4 pages ... So no, that way of testing for backlinks is rubbish.