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From what I've observed in Adsense targeting... and I don't do Adsense myself... co-occurring words are used much more than they would be in competitive SEO.
You can get ideas for other articles using Google's Keyword Tool.
I'd be careful about dropping in too many synonyms based are the Google Keyword Tool, but used with restraint, this might not be a bad approach.
What you need to do is to describe the subject you're targeting, in natural language as longen suggests, not just drop in some keywords. Natural language will give you the natural co-occurring vocabulary that you're apparently missing.
An example of co-occurring vocabulary... if you wrote an article about the White House, it would be natural to expect words and phrases like "president", "united states," and "washington dc" to appear in that article. You might have an article containing the phrase "White House" that doesn't contain these words, but it would be strange to have an article about the White House that didn't contain them.
In your case, the keywords you're using are apparently natural co-occurring vocabulary for articles in the subject area of the phrases being matched up with your article.