If you had an 80% success rate but were 18 for 20 you'd be some sort of wizard. :))
Anyway, I think the author is assuming that each time you apply, you're applying for multiple jobs, getting multiple offers, and turning down all but the best one. I was a little surprised that my score was so low, too, given that (as a junior) I've only had 2 on-site interviews but have received 4 offers, which gives me a score of 60. Meanwhile, interviewing for 100 jobs and getting 60 offers gets you a score of 120.
Anyway, I think the author is assuming that each time you apply, you're applying for multiple jobs, getting multiple offers, and turning down all but the best one. I was a little surprised that my score was so low, too, given that (as a junior) I've only had 2 on-site interviews but have received 4 offers, which gives me a score of 60. Meanwhile, interviewing for 100 jobs and getting 60 offers gets you a score of 120.