It may feel random, but there's no denying the process effectively ensures that only people of the top calibre and cultural fit enter these firms. Yes, who you get and how they treat you is random, but that is stylistic and in no way indicative of how they rate you.
There's three reasons the perceived unfaireness/randomness of interviewers doens't translate into true unfairness of your results:
1) There is a standardized, incredibly previse and clear rubric that each interviewer fills out - this quantifies your abilities across key areas and there is very little room for interpretation in this.
2) Some tough interviewers are actually lenient and some nice interviewers are actually tough. I've known people that are sweet as sugar and have every candidate walking out thinking they aced the case. I know others that have the opposite effect. How a perosn treats you isn't the indication of how well you did - it's how they treat you in comparison to how they treated others. Think Simon Cowell: a "jerk" but knows talent when he sees it.
3) This leads to my third point - the interviewer you were given interviewed lots of other candidates that day. If they were "easy" they didn't give everyone passing marks, and if they were "hard", they didn't block everybody. Each interviewer, regardless of their style, still ranks you in comparison to your fellow interviewees - there's no unfairness in relativity.
I'm sorry, but you did not do as well as you think you did. It sounds like your performance is the equivalent to passing a class but by no means equivalent to you graduating Magna or Summa Cum Laude, which is what you need to be.
You can get there, if you work hard and have the right mindset - but "bad luck" is not the right mindset.