My experience is that people tend to have fairly similar patterns of behavior. If they taught themselves a relevant new skill at their last job, they will probably teach themselves a relevant new skill at the next one.

Google themselves admitted that they basically don't know how to interview well, which tends to suggest that style of coding question doesn't work especially well. You have some people who have faked y (false positives), and others who could very easily figure out y given a bit more time but don't do well under pressure (false negatives).