Why A registration required meeting still need participents to sign in?

I have a paid account, I set up a registration required meeting, and after everybody registered with an email address and a unique question, they are still asked to sign in or via the link, but I didn`t enable the "Require authentication to join" function. 

Could anyone tell me why and how to solve it?

And also, when they registered with an email and unique a question, I would expect them to sign in with those two matching questions but seems they don`t have any connection. Why can`t they just join with the right email address, the right passcode, and the right unique question？