NPS Managers Directed On How to Evaluate Employees

I know back when I had a regular, 8-5 job, I looked forward to my annual reviews with a bit of trepidation. I never knew whether my manager was going to be in a good mood when he got around to that delightful task.
National Park Service employees no doubt will be joyed to hear that a directive from on high has taken away some of the speculation as to how their evaluations will be pieced together.

Park Service managers have five categories under which to rank the performance of their employees: "Unsuccessful," "Minimally Successful," "Fully Successful," "Superior," and "Exceptional."
Now, under the agency's new performance rating system, park managers have been given specific guidelines as to how, and how many, of their employees may be given "superior" or "excellent" ratings.
Here's a snippet from a memo one regional director sent to all the superintendents and associate regional directors under her/his umbrella:
"....We wanted to be sure to caution you on what levels of performance you are identifying for various staff in your organizations. All exceptional ratings must be approved by the next highest level supervisor before the rating is communicated to the employee. The Department has also cautioned us that 'superior' ratings should be few and 'exceptional' ratings should be rare.
"They do not want to see this new appraisal system migrate into the former situation where only an 'exceptional' rating was considered satisfactory. ...'Fully successful' is a good rating that still allows for performance awards if you chose. The other ratings are on the upper and lower end of the bell curve and should be given out for only those employees that have gone far beyond their standards or far below their standards so please pay close attention to the ratings given out in your organization."
Now, if you've been paying attention, you know that top hires/promotions in the Park Service have to pledge a loyalty oath to not just the Interior Department's mission but also President Bush's environmental philosophies. And now there's an added layer of performance review before top-notch employees can be recognized for their abilities and on-the-job performance.
One question that comes to mind is whether this directive mandates that employee evaluations be administered in a pure bell curve. Another is whether a superintendent's "superior" or "exceptional" ratings will be smacked down by someone in the regional office for reasons based on matters other than actual performance.