Earlier quoted context omitted.
The little captcha box does some processing before deciding what to show you. If you look suspicious, it can show you a more complex challenge. If you look like a normal browser, it might show you a house number to read (to improve its Maps product maybe). If you already have a cookie set because you already proved you're human, maybe it's just that check-box that says "I'm a human".
Unless you're able to explicitly state how this is done and/or how to trick it in think you're "high risk" - then seems like speculation; yes, I'm aware Google's said this, but never seen an proof of it. I've found bug in the past in the system that were easy to fix, told Google, but the bugs never were fixed.
Try doing a Google search from tor. The catches that tor users get are borderline impossible because of the suspicious ip and/or browser.