I'm pretty sure that is how it works. Do you have anything to support your statement that the rankings are based only on the top 20 commits? Here's an image tweeted by Shannon Terry, 247Sports Founder/CEO:
To quote, "c is a specific team's total number of commits." It doesn't say "c is a specific team's total number of commits up to a maximum of 20."
And, if you go in and play around with the class calculator, you'll see that dropping players from the bottom of our class reduces our class score. Right now, we're 30th in the country with 24 commits and a class score of 192.18. If you pretend that the bottom 4 didn't happen, it puts us at 189.37, which drops us down to 31st and barely ahead of wisconsin at 32.
The bottom recruits definitely don't move the rankings as much as the top recruits, but they do impact the score.