nobody knows for sure how Google count the backlinks and what kind of backlinks they display. But I am sure the age of the backlink is pretty important for Google
Google only shows a random sampling of the links they know about. They certainly know about more, but only show a few. Also like John said the different engines use different methods to determine what is and isn't a backlink and quality of the link is particularly important to Google.
What I have noticed that Google mostly lists the old backlinks from relevant websites. Yahoo don't count links correctly, they always show more than the actual, don't know how they count backlinks.