Simply put, it's pretty much a people based lottery.
It very much depends upon how you feel on the day, how the interviewers feel on the day and how the rest of the candidates feel on the day.
Yes there are warts in the system and yes the company is well aware of them but, the simple truth is, that is the 'warts' are the same for all candidates then that makes it, fundamentally, fair.
That is all the company wants, is a fair and equal selection process. It's human nature to have good and bad days and if you happen to 'luck out' then great but if you don't then unlucky but that sums up a 'day at work' really.
Can't ask fairer than that can you.