From feedback from webmasters and my own use it for well over a year now it has been apparent ReCaptcha has been exploited in some way-
https://www.vbulletin.com/forum/show...-to-Combat-It?
My theory is the bots are copying the image and displaying it on file-sharing websites where thousands of people eagerly type in the answers every minute of the day.
That's one guess.
I've also seen a video showing that the vast majority of the time you really only need to type in the "easy" half of ReCaptcha for it to go through- in which case basic OCR can be enough, when coupled with multiple tries..