I’m really not sure your confusion. Hall has an expensive cap hit and would need to be retained at 50 % to have standalone value. He also isn’t that good. Doesn’t kill penalties and is basically a third line/PP2 option for a good team.
The third they got is their own third previously dealt. It’s closer in value to a contender late 2nd than a contender late 3rd.
Hawk GM presumably kicked the tires around to see what Hall was valued at. Likely word on street was a contender late third for Hall at fifty percent. If a late second for Hall at fifty percent was available they probably would have just done that. I suppose could have waited around until right at the deadline if hall had a hot streak and returned more. He could also get injured and return nothing.
The Rantanen piece is them retaining on Rantanen’s more expensive expiring cap hit than Hall’s expiring cap hit. So they retain a bit more, move Hall’s full value instead of half and end up with a pick roughly twenty picks higher.
It’s really not that complicated.