Have you ever been around a friend who points out a flaw about themselves and you say, "I would never have noticed that if you hadn't pointed it out?"
Google is doing the same thing sans flaws.
On the Official Google Blog, Ben Gomes explains that sometimes Google's search experiments are very subtle. They'll test things like white space in the search results or how big the plus box should be next to a stock quote link.
The differences among the options being tested are very difficult to detect without being told what's going on. But the results of tests show that even a small number of pixels can make a big difference. For example, it could mean the difference between a searcher spending a lot of time on one search result - and not necessarily the first one - or a searcher being able to scan the results more easily. And then Google has to decide which is better.
All in all, Google's search experiments are not always about the alogorithm. What do you think of these experiments? Leave your thoughts in the comments!