My experience with those big companies and those tough but allegedly fair review processes is that management has no idea who is actually doing the work, and disproportionately compensates people they like on a personal level, unfairly. The bias reenforces itself when the "top performers" are preferentially assigned prestigious work that can look good on the next review. You can tell who the stupidest people in the organization are by the faith they have in the fairness and appropriateness of the review system.