A Google Groups member notes that Google seems to be able to stem complex plurals as of late and accurate suffixes. For example, if you search for "accredit" or "accrediting," you will see "accreditation" highlighted as well.
Forum members also observe that Google is "capable of ranking on synonyms now" from an example posed by a user who said that Google is highlighting phrases that is are not at all tied to the on-page text.
One forum member says that Google must be pulling this data from a huge depth of information -- probably a very comprehensive dictionary, if you will.
Honestly... I'm not sure if Google uses links anymore to determine relevancy... I think they just build a huge world wide word density cloud and use that to abstract the depth of information for a search term based on the frequency it appears within a community...
JohnMu from Google adds that this may be related somewhat to personalized search results.
The discussion is interesting and definitely is a good read for those interested in semantic search.
Forum discussion continues at Google Groups.