There are many things we do not understand about hiring. However, our inability to predict the future does not mean we cannot learn as much as possible about the present. The only way to do that is to:
1) conduct our own research, or
2) read research conducted by other people.
We may not like to believe research results because they interfere with personal opinion, but research is the only way to know for sure which recruiting and hiring tools work best. A hiring manager might be 100% convinced of his or her opinion, but that is not the same as being 100% correct about the candidate.