The simplest explanation would be that every cost center/P&L had to offer up 6% of its people, regardless of that cost center's overall trajectory, place in company strategy or open headcount. And that every cost center's VP or general manager or whatever just got collared by HR, was given a list on a piece of paper and couldn't leave until they chose 6% by ...whatever metric they came up with on the spot.
Whatever actually happened I'll never know, but what I've seen was 100% compatible with that theory. Which in turn looks pretty inept.