The ratio of received merits per post totally depends on post quality. The higher your average post quality is, the higher the ratio is. There are some users whom have total merits (all earned or from airdropped + earned) outweight their activity points. There is even a @nullius, whom earned 777 merits, with total postcount is 928, and activity point is 168. If @nullius was actively within last one year, he even would have ranked up to Hero Member.
In contrast, some users can make hundreds of posts and have not received a single merit.