Keith, there are a lot of factors that go into the actual search engine rankings that you and I see on the search results page. With the particular query "what is google voice", Google has decided that the MacWorld article, with a title of "what is google voice" is more appropriate for the query than the actual Google Voice page.
I would gather that from the query that the user is wanting an explanation of the product: they're not wanting to go to the Google Voice page itself. So, Google is showing the answer to the question rather than the actual site that they're asking about.
This also is a good example of Google Hummingbird at work, where they're able to understand the query better, and deliver better search results based on the actual query, not just look at "keywords".