Google and other ads are specifically designed to look like search results and exploit the fact that older people cannot see contrast of the background as well as younger people.
The contrast on the background is much lower than the federal 508 standard for contrast and I think has changed to over the years to a lighter shade as Google "optimizes" it.
One is an ad and one is a search result, is there much difference? Given the average quality of monitors, I think those are designed to fool even otherwise sharp eyes.
As has been mentioned in other posts above, it seems doubtful that ad companies (at least large ones) want these ads to be deliberately misleading.
If you click on an ad that isn't relevant to you, there are three major harms caused. One, your time is wasted closing it and going back. Two, the advertiser may have to pay extra for a useless click. Three, Google (for instance) has to consider the likelihood that this is a useless or fradulent click, and possibly reimburse the advertiser if so.[1]
All of these are bad for Google (for example). The offset gained by a few dollars in revenue is probably not going to counter that, because what they really want is users to continue using their search engine and they want advertisers to be happy with the clicks they pay for. Those are really important for them to keep. Tricking you into clicking useless links is bad, bad, bad. (Adwords hosted on someone's website is another story; that person might not feel the same way.)
But if, on the other hand, the ad actually is relevant to you, then it's great for all three parties if you click it. So I'd hesitate to attribute these UI choices to trying to fool people.
Disclaimer: I can't remember the last time I clicked on an ad anywhere on the internet, except by accident. But that doesn't happen to me on Google or other reputable advertising-based sites.
I bet it's related to the angle of people's screens: my laptop's screen is not directly pointing at me, and the samples in your screenshot are nearly indistinguishable from one another -- but is very visible when I move it to my primary screen which is at a different viewing angle. I first noticed the issue when I was looking at a graph with a watermark, and the watermark was more visible than the graph (until I looked at it on my second screen), so I'm sure it's relatively common.
On a CRT this isn't a problem, but I imagine that many people have their LCDs at an angle which isn't perfect for viewing such contrast differences ... and if a user (like my parents) are not as accustomed to playing "spot (and ignore) the ad" based on content as we are, then they might completely miss the visual cues as well.
Also, age is big differentiator for seeing contrast.
Getting people to click on the ad instead of the search result makes Google about $20 per click because of the keyword, so no wonder it's designed to be almost invisible.
The contrast on the background is much lower than the federal 508 standard for contrast and I think has changed to over the years to a lighter shade as Google "optimizes" it.
http://i.imgur.com/Wmdd0.png
One is an ad and one is a search result, is there much difference? Given the average quality of monitors, I think those are designed to fool even otherwise sharp eyes.