The issue with tricky algorithms questions is that they often become tests of how well you can keep your cool. I've (and I'm sure others have also) bombed relatively easy questions due to nervousness compounding. I've also done much better than expected on other interviews because I remained calm and kept the interview flowing smoothly. This is the real skill under test, IMO. Of course, your ability to solve problems is still important, and a subset of what's being tested. The issue is just that there's a lot of (possibly inherently random) factors that cloud that signal.
My favorite example of this is when an interviewer asked me about an algorithm. In the course of the problem it occurred to me, what I needed was to break down the input integer into two parts--two equal parts that could be multiplied together. I kept referring to these two parts as such.
After the interview he said I was clearly weak on math because I didn't know the definition of a square root. Something I did know (10 or so college math courses in linear algebra, calc etc) and was describing, but was at a loss for words.
