Most of the time Adsense displays ads based on the “user interest”, this means that whenever someone uses Google Search, Google will store a small interest based file (user last search etc.) as cookie, so the next time that someone uses Google search the ads will be based on previous interest or search!
For example you have a “SEO tips” blog, before arriving to your site someone might have searched for “best video games”, this data has been stored on that someone's computer in the form of cookie, now that the same person searches for “SEO tips” and visits your site the ads that the person gets will be based on “best video games” keyword because of the previous search interest (cookie) that has been stored in the computer. This is known as not relevant ads because the ads that users get are not relevant to your topic neither to your content, and this is really bad because it affects CTR and your overall earnings in a negative way, but don't wary there is a way to turn this function off!
Go to your Adsense account and click on Allow & block ads then click the Ad serving tab, under User-based ads uncheck “Interest-based ads - show ads based on user interest categories, and allow the use of visitation information from my site to help create interest categories”
Good Luck and have fun!