Because Person A is paid more than B, is older, lacks a critical skill or difficult-to-replace experience, has less seniority, has distracting family obligation

Because Person A is paid more than B, is older, lacks a critical skill or difficult-to-replace experience, has less seniority, has distracting family obligations where B is single, or maybe A just doesn't have as warm a personal relationship with the manager as Person B. There are plenty of bad reasons A might get the boot. And that's assuming the manager has any discretion. Often it's whole departments that get cut, for purely financial reasons. Considering how many posts there are on HN about how difficult it is to measure performance, I'd be skeptical just how well the average company can rank performance, especially amidst the stress and time constraints of a large layoff.