My speculation (I have no data) as to why it works: Showing this to everybody shows it more reliably to those who actually will sign up. It is hard to target t

My speculation (I have no data) as to why it works: Showing this to everybody shows it more reliably to those who actually will sign up. It is hard to target the right people reliably (maybe they deleted their cookies, maybe they use a different device...) so if you just show it to everybody the right people will definitely see it. First time visitors still won't sign up, but significantly more established readers will.