All of the content that is contributed to the Stack Exchange sites is made available for others to use - http://creativecommons.org/licenses/by-sa/2.5/ thus we own the content rather than the site (good news!).
The downside of this is that other sites can copy that content and publish their own site covered in adverts and try to make some money for little/no effort.
Theoretically these copycat sites shouldn't feature very highly in search engine results as the license for usage require attribution (a link back to the original source). Unfortunately these sites did start gaining ground in search engines and after a lot of head scratching one of the measures put in place was to add keywords to the
For more details on all of the above see this blog entry: http://www.codinghorror.com/blog/2011/01/trouble-in-the-house-of-google.html
For a bit of extra info on keyword significance see: https://webmasters.stackexchange.com/questions/6556/does-the-order-of-keywords-matter-in-a-page-title
As for your actual question; Why are some questions displayed differently from others?
I can not answer authoritatively on this however I believe the keyword that is used is the first tag that isn't in the question wording itself. I have found exceptions that disprove this rule but it seems to apply in most cases.