People tend to forget that AdSense, just like Google Analytics, knows where visitors were referred from and what keywords got them to that site. Basically, if you have visitors being referred from off topic content pages, AdSense knows this and probably devalues those referrals heavily. This could work heavily against your AdSense earnings if you're just link building from any old junk domains attempting to raise PR, and could also negatively impact your SEO efforts in the process.
Just think about the content and quality of where you establish links as it's kind of obvious to me that the location of those links establishes intent. Just like Panda and Penguin use that information to push your sites further down in the SERPs, there's most likely an AdSense Smart Pricing counterpart that uses similar information to reduce your payouts even further.
I think this is kind of evident based on all of the threads where people keep claiming they have tons of back links and lots of traffic yet their earnings are getting lower and lower. There are obviously a bunch of other reasons that the payouts decrease when traffic increases, standard supply and demand stuff, but it shouldn't cause an overall earnings decrease, just spreading it out a little thinner. Actual earnings decreases would indicate to me something has caused the site to be devalued so it's either the content or the back links. If the content hasn't changed as many claim, and Google hasn't changed how they valuate your content, that just leaves the back links.
In other words, bad back links could spell AdSense bankruptcy!
I actually think whatever changed was implemented in AdSense about 2 years ago.
Talk among yourselves.