Departments usually act as advocates, trying hard to send up unanimous votes. But when a few of the votes are negative, deans are likely to think that those votes really represent twice that number who are lukewarm about a candidate. Second-guessing is especially common at universities, like N.Y.U., that are striving to improve themselves. Other evidence in tenure files is also unreliable. Outside letters are rarely as honest as they should be, because their writers don't want to risk alienating a colleague.
Many academics refuse to write evaluations unless they know in advance they can write positive ones.
So deans in turn ask who was invited but declined to write. Clever department chairmen counter by asking people informally if they would write an evaluation before they turn to them formally, so that official turndowns are rare. Evaluators know their letters will be scanned by skeptical administrators for any minor qualifications, so they give none. One of my outside letters disagreed with some aspects of my work. For the most part, I was told, he praised me strongly, but nonetheless mentioned some of my research and arguments he found problematic.