alt Hacker NewsRecaptcha contains a whole maximally obfuscated virtual machine with its own bytecode language. It measures your mouse movement, clicks, timing, cadence, hesitation, consistency, tile clicking order, etc.
Ambiguous tiles are deliberately placed because the behavior they elicit from humans can be used to discern them from bots.
Yes, the "correct" reaction to the ambiguous tiles is to hover a bit indecisively. You need to waste a certain minimum amount of time on the CAPTCHA. I've found that applying videogame reflexes and zapping all the tiles in a short period of time is a fail, even if they're the correct tiles.