Very interesting question, thank you very much for your feedback.

What I try to say here is: 50% of all rewards go to posts below $6, and 50% go to posts above $6.

Then $6 is this middle point. It corresponds to the median if we define the population as the number of rshares (not the number of posts) in a discrete probability distribution (wiki ref).
Also you can see here https://joticajulian.github.io/steemexplorer/#/reports this distribution. I draw the median with a different color.

This value is very related with the previous hardfork, where we had linear rewards: Posts above $6 get more rewards compared with HF20, and posts below $6 get less rewards compared with HF20 (not taking in consideration the cut of 10% for sps).