That calculator is super helpful and explains the logic in selecting the field. You can see the selection details and common competitor chains. If that algorithm is correct....
The South Central is definitely getting 4 bids due to Lewis and UCCS doing well at Parkside and Lewis. The teams in front of them will rack up a bunch of secondary wins through them if they don't already have direct wins.
In the Central Region, Washburn looks to have been the only team that raced outside of the region and beat #8 South Central (MSU Denver) and MSU Denver did not race anyone outside of the Dallas Baptist meet. MSU Denver only has a bunch of direct losses to South Central teams and no quality wins vs. teams outside of the region. Same can be said for the rest of the Central Region (minus Pitt State, who will get an auto). That is what is hurting the region from what I can see and according to College XC Page and looking at schedules/meet results.
For the Midwest argument
#4 Midwest(Walsh) has a direct win over #6 West (Biola) #4 South (Flagler)at Lewis. Indirect win over #4 South (Embry Riddle) via Tiffin
#5 Midwest (Ashland) has a second degree win over Biola, Flagler and Embry Riddle by defeating Tiffin at conference. It doesn't look like they have any quality direct wins over any teams that would be in the national qualifying conversation outside of the region.
#6 Midwest (Tiffin) has direct wins over Biola, Flagler, and Embry Riddle. They have no direct losses that will hurt them finishing #6 in the Midwest. Their only key losses are to Ft. Lewis, Alaska, and Northwest at Lewis and teams that are projected to get an auto qualifier.
#7 Midwest (Michigan Tech) has direct wins over Biola and Flagler at Lewis. They won at Griak but didn't look to beat anyone in the national qualifying conversation this year.
Biola doing poorly at Lewis will hold the West Region back if they do well at regionals.
From what it looks like there are 12-13 teams vying for 10 spots. The only way it changes if any team lower than #7 South Central #7 Midwest and #5 West jumps into one of those spots or higher. It would open the door for Flagler and Embry Riddle.
Just my opinion but maybe D2 XC Stats is seeing something I am not.