A good reference remembers the conditions under which someone did their best work.

Judging a person without their circumstances gives you half a description. Somebody who flourished under one manager and struggled under another has not changed in ability between jobs, though the conditions certainly changed. A reference worth reading says what a person needs in order to be good, not only how good they were.

When you speak for someone, describe what let them do well rather than delivering a verdict. When you are described that way yourself, notice which conditions keep reappearing. Knowing what you need is far more useful than knowing your rating.