Google’s Matt Cutts posted a video answer on the question about why and when Google will ignore your title tag and use something else for the snippet title in the search results.
Matt explains in the video that Google really wants the title of the snippets to match on some level the query of the searcher. This logic often results in a higher click through rate on the URL and thus should be better for both the searcher and the web site owner.
The criteria Google uses when coming up with a new title tag are:
(1) Something that is “relatively” short
(2) Have a good description of the page and “ideally” the site that the page is on.
(3) And that it is relevant to the query.
If your existing title tag fits the criteria, then Google will most likely use your title tag. If not, then Google may use (1) content on your page, (2) anchor text links pointing to the page and/or (3) may also use the Open Directory Project.
