Yes sorry it's confusing terminology, what I meant is they get paid regardless of your number of paid subscribers. The price tiers apply to both free and paid readers, so if you pass 1000 free subscribers you move up to the next tier. There's no major incentive for them to encourage readers to pay authors, that's up to the author.

See the FAQ (https://ghost.org/pricing/): "Every user with an active account to subscribe to emails or login to your site counted as a member, whether free or paid."