Actually I don't have that problem. My top performing article is a huge long page of wide-ranging text, and it makes me a fair bit of cash.
My worst performers are short essays that argue against one thing or another, that explain what it's a bad idea for you to buy widgets, and so Google serves ads that tell you where to buy widgets. It's not able to discern that the keyword appears in a sharply negative context.
If you sticky mail me I'll give you a URL with a specific example. I've been taking adsense off most of these pages but I'm going to leave it on that one so others can see it.
A hypothetical example would be an essay arguing the benefits of vegeterianism. Discussing the evils of meat consumption would likely prompt google to serve ads for steakhouses.