The fact that the same person going through the same process twice gets a completely different result strongly suggests that luck and randomness is a large factor, despite the apparent thoroughness and time investment.
I've advised people on a job search that the way it works with tech hiring is they ask you a few questions and either you'll happen to be able to get the answer quickly or you won't, and if you go on enough interviews eventually you'll get one where you get all the answers quickly and you will look really smart as a result. Consequently, you can't pin your hopes on or predict whether you would be able get hired by any particular company.
Randomness is a factor whenever you have social interaction; as it turns out, people are subjective to the core. Trying to gauge someone's ability to deliver results is educated guesswork at best. You can only be so objective about it; there's going to be some measure of variance.
Google errs on the side of rejection because one bad hire has a much bigger impact than one good hire.
Yes, I've said it before, Google is what you get if you hire essentially at random and then tell all those people that the process that hired them is infallible.
I work for Google, and no, we do not think our process is infallible since we understand there is a large chance for false negatives and even a small chance for false positives.
We do not hire at random, each hiring goes through multiple interviews and the results of each of those interviews are also reviewed by multiple people (interviewers are required to record all notes/code produced during the interview). Then a decision is made. If we feel not super certain we err on the cautious side and turn down the candidate, even though we know there is a good chance for false negatives.
As far as I can tell, our false positives rates are very low, and everyone I've worked with here at Google are incredibly qualified at their job. Are there people who don't perform? For a company this size that's an obvious yes, but I think if you judge interview's goal of eliminating false positives at the expense of producing false negatives, then we've been pretty successful.
I've advised people on a job search that the way it works with tech hiring is they ask you a few questions and either you'll happen to be able to get the answer quickly or you won't, and if you go on enough interviews eventually you'll get one where you get all the answers quickly and you will look really smart as a result. Consequently, you can't pin your hopes on or predict whether you would be able get hired by any particular company.