I've been through the Google interview process [I'm not going to blog about it with any sour grapes, I'll try again for a different position later.], and I realized afterwards that it was a lot like map-reduce. You're being thrown through a bunch of interviews by different people (the map phase) who then forward their reports to their hiring committee (the reduce phase). Every interviewer is seeking to do a similar kind of analysis on you: are you worthy of hiring? Then they gather it all together and grind the sausage. They probably interview thousands of people to harvest enough talent to fuel their progress, unlike other organizations that only hire to fill known needs.
They don't tell you why they approve/reject, but I imagine that they probably have a similar process on the other side if you are hired with regards to the package you're offered. Very likely, they'll seek to optimize the right amount to give you.
And yes, I do think they're probably missing out some great talent, because not everyone can scribble it out on a white board on-the-spot. But you have to be able to communicate effectively to get anywhere, even in Google.