How can I tell what makes AdSense ads irrelevant on my site

Google is giving personalized ads. Period. You cannot turn that
off. It's the wave of advertising.

Here's how you can "help" get more "targeted" ads. I use both
of those terms very loosely.

1) Get visitors to surf as many pages on your site as possible.
The more they surf, the more they show an interest in the
topic of the site. Also, get links on similar pages. A person
surfing from one site on loans to another, now has recent
record of searching for loans.

2) Watch the words you emphasize whether on purpose or not.
Watch what you have in your page title. Watch the words you bold.
Watch what words are near or at the top of your page. Watch what
you have as h1. Any heading size, will be emphasized. Watch the
size of your article. The more words, the more muddling. Did you
know google tells you to make articles short and sweet?

You cannot really get around personalized results, and, there is no
reason why you should. People are people. Google has been at
this for a long, long time. The ads you see are not what others see.

Here's another one that takes more testing. Some ad unit sizes attract
a ton of advertisers. The more advertisers, the more potential of getting
less targeted ads. But, your cpc may go down. Google says that more
advertisers use 300x250. Test all sizes and use 3 ad blocks.

Paul