I would guess they have a system such that after a user has passed N captchas successfully, they trust its a human and start displaying them (a portion of) unlabelled captchas that will always succeed and that's when novel labelling happens.
Or something along those lines. And then you can get creative displaying same captcha to multiple users, etc...
Or something along those lines. And then you can get creative displaying same captcha to multiple users, etc...