The one factor that may contribute to what you are seeing is the ratio of contextual to interest based ads served
I believe that is larger factor than perhaps we might imagine. I have been saying for ever since those ads came into being that they are all too often totally pointless.
Over 90% of AdSense ads I see on sites I visit are ads for items I have either purchased on eBay, or have placed on my watch list, or from my trusted suppliers. These sites also are sites nothing to do with my recent purchase of an all singing, all dancing CPU.
The hysterical part is always, every time, without fail - the part offered in the ad will be priced between 25% to 100% dearer than I had already paid.
Pointless, the publisher would never get a click, except from the less discerning buyer.
But, Mr.AdSense and others swear by IBA - who am I to disagree? The hard part for the hapless publisher is they have no real control.
Yesterday! Imagine one or more AdSense ads from PC suppliers for an Intel i7-6100K CPU on a recipe site when searching for a "Coffee Cake" recipe.
The recipe was 'triffic, the CPU beyond my budget.