Search term not in page summary
 

Does anybody know why the search term doesn't always appear in the summary of the search results?

My example is http://development.itsystems.co.uk/h...tring=Building

You'll see from this that the search term is "building", but this word is only highlighted in some of the search results. Whilst the word does appear in all pages, surely it's fairly important for the visitor to see the context of the word when determining the relevance of the page?