When someone visits your hubs they might click on a referral link that takes them to a sign up page or click on a link that takes them to another one of your hubs depending on how you've set up your links. Or they may follow a link in your profile to a sign up page.
Finding out that your sign ups have written hubs of their own is significant.
The way you can find out who actually used the links to sign up is by going to Earnings, then Referral Trackers - This shows your trackers and if you have sign-ups and also how many hubs under each of the trackers. Click on those trackers showing hubs have been written. This gives a list of those people who signed up under your tracker. Clicking their link takes you to their profile with nothing more about revenue earned.