Targeting the ads by keywords

At this time, AdSense only targets ads based on overall site content, not keywords or categories.

If you'd like to display ads related to specific topics on your website, we recommend including more text-based content about these topics, including complete sentences and paragraphs, to assist our crawlers in gathering information about your pages and determining relevant ads to display. Please note that it may take up to 1 or 2 weeks before any changes you make have an effect on your ad targeting.

SOURCE:  google.com