There was a discussion about this a year or 2 ago.
NASA was ranking No.1 in Google for the keyword 'home'...
Many of the world's
SEO experts, while toying with the question "what makes a website rank?" (as an
SEO is prone to do) came to the conclusion that the major contributing factors were Onpage. apparently having a PR10 will cause you to rank very high for any keyword that is on your page. Try typing "Image of the Day" or "multimedia" or "Interactive Features" into Google. NASA is nearly always amongst the top ranked websites for for keywords on it's homepage. That fact is not because they have links pointing to them with those specific keywords.
With regard to semantics...
Semantics is not purely linguistic.
Semantics is also philosophical.
Have a look at some of the Google patents.
Google has patents involving algorithms that measure how long people stay at your website, what they do while they are there, how many pages they look at on your website, where they go when they leave your website, how they leave your website...
As far as Google is concerned (and other search engines) there is evidence that they are trying to divert from linguistic semantics and make use of 'relationship semantics" (or philosophical/logical semantics) and reactional behavioral semantics.
Read:
Semantic Web Patterns: A Guide to Semantic Technologies - ReadWriteWeb
Chief Marketing Technologist by Scott Brinker: SEO + Semantic Web = SEO++