5/30 cap hit is what the player counts on the cap as of May 30.
9/10 cap hit is what the player will count on the cap on September 10 if they make the 53-man roster after the roster cutdowns.
Slightly more specifically:
The anticipated guaranteed signing bonus (based on Miguel's analysis of historic bonuses at those slots and escalation of the rookie cap) will count against the cap whether the player makes it or not, and so assuming they will that portion of the bonus counts for salary cap accounting during the offseason.
At the end of training camp, the rules change and each team has a roster of 53 players, a practice squad, and so forth. The Sep 10 projection is what that player will count against the cap if he makes the team.
In the question above:
The actual cap impact of the rookies is uncertain. Some will make it; some won't.
The question of 'what is the salary cap impact when they are all signed but before the season starts' is something a highly knowledgeable analyst can project. Miguel has done the research.
There are some different questions that are being asked that may sound similar.
What is the total anticipated salary of all the draft picks? That's Miguel's figure of around $6mm.
What is the projected net impact if all those players make the team and replace exactly the 11 least-expensive other players? That's the ~$1.6mm figure. But that really sits in a range, depending on your assumption of who is cut and who makes the team.
What is the amount of cap required to sign those 11 players? That has much more clear assumptions, and that's the question Miguel is answering.