reCAPTCHA relies on things like Google cookies to lower the "user is a bot" risk score. Higher risk scores (such as when you go via a blank slate browsing session) result in more/more difficult challenges.
That's just code for "it rejects correct answers to frustrate you." If you manage to get the noscript version of the captcha with otherwise the same browser state it will accept a correct answer the first time nearly every time. Presumably this is because they didn't bother to implement their "hassle the user" code in the noscript version; it's probably neglected by google since it's disabled by default.
For instance, the sloooow fade in of challenge tiles... what legitimate purpose does that serve? That's not there to make it harder for bots. That's there just to hassle and punish real humans that google dislikes because they don't buy into the google 'ecosystem'. The more they dislike you, the slower the fade in gets. The fade-in can be several seconds long in severe cases.
I run a combination of uBlock Origin, Privacy Badger and Firefox's tracking protection. Can confirm, tiles take 5 seconds to fade in, I have to do 3-5 rounds of it, and unless it's really important I'll just tell reCaptcha to piss off.
