In XOM system, 50% of senior executives are ranked in the bottom half. The forced ranking works the same way. This senior rank group has zero professionals in it. At that level, 70% of compensation comes from cash bonus (20%) and Restricted stock (50%). bottom 20% get none of these incentives, so when executives drop, compensation takes a major hit.
Note: there is a 50% chance you work for a bottom half executive and a 20% chance you work for a Needs improvement executive.