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.
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.