A number of my boss's reports had landed in the achieves category for all of 2014, but nonetheless received a communication from HR this past April that they were in the bottom 5% of the company for 2014 and at risk for termination if their performance did not improve. This surprised my boss and the reports.
Obviously if you're in the bottom 5% you're well below average. The question is how you can consistently be ranked in the middle category yet nonetheless be in the bottom 5%. That would seem to make the quarterly ratings a rather pointless exercise for communicating to employees what their ongoing performance is.