So it's mostly about stars.
Pretty much. It's not an exact science, but those numbers don't lie. Particularly when you consider the number of 5 star players per year (approx. 25), a very high percentage of those are drafted in the first 2 rounds.
It's kind of interesting because it mainly comes down to how many of each ranking *should* turn out to be correct. It appears from the graph that about 25% of the first round (8 players) and about 12.5% of rounds 2 & 3 (8 players) were five stars along with about 6 others drafted in the last four rounds. So - assuming the number of five stars in each recruiting class is the similar to the number of five stars who are draft-eligible - about 67% of the five stars were drafted in the first three rounds and about 90% were drafted at some point. So they hit on the five stars pretty well.
The four stars is where it depends on what you would expect. It looks like there were about 170 four stars drafted (approx. 22% of each graph). There are almost 300 four stars each year so about 55-60% were drafted. So that wouldn't be much better than a coin flip as to whether the four stars turned out to be good enough to be drafted. Lower-rated players were drafted almost three times as often as the four stars. Obviously there are a lot more of them but I think that also shows that there isn't as much of a talent gap between the second half of the four stars and a lot of three stars as some might think.
I don't think your numbers are at all right, except for the 1st rounders, though your conclusion seems more right than you think.
First, there were 256 total guys drafted.
From the article I count 21 first rounders total, or 84%. (21/25)
It looks like 58 4 stars, if you read the graph as 7 in the first round and count up the rest from the article. Not sure how you come up with 170. And there weren't 300 4*, they are top 300 but you have to take out the 25 5*, which leaves 275. 58/275=21%
You really have to finish the math on 3* before you can draw any conclusion like you did. My initial impression was that it is ridiculous to compare the "second half of the four stars" with three stars because you're taking ratings from pre-college, then evaluating the college careers of the 4* and taking the underachieving bottom half and comparing them to another set from pre-college rankings. Not a valid comparison. Also I had no idea how many 3*s there were so drawing a conclusion without showing the same type data doesn't help. But let's look at the 3* numbers.
I count 115 3* reading the 1st round as 16 (half) 3* and adding the other rounds. 3* is top 750, take out the 300 5&4* and you have 450. 115/450 = 25.5%. This is shocking to me that 3*s fared better than 4*s. I wonder if this was an aberration year.
I don't find any data on how many players are ranked 2* and obviously the pool for unranked is huge, so one can't draw any conclusions on those numbers except to say that a 2* can certainly be drafted, even in the first round,
http://www.cougcenter.com/wsu-football-recruiting/2013/2/5/3956800/rivals-scout-espn-247-star-rating-system-national-signing-day is where I got the 25/275/450 numbers for 5/4/3* 247 ratings.