I can only theorize, but my guess is that a human signs up, then allows the bot to post. I get this quite often from clients where they have an online form with a reCAPTCHA, and they are getting regular spam. I have to explain that some bots are able to figure out the reCAPTCHA, as well as let them know that sometimes real humans are just paid to fill out forms and bot detection isn't going to help in those cases. This is especially true with services like Fiverr.