We agree the words boon, noon, moon, and snood all share the one-and-only \ooh\ pronunciation; but the evidence on room and roof tells a different tale. In fact, both \room\ and \rum\ are considered standard, variant pronunciations and so are \roof\ and \ruf\.

What sets the standard for a standard pronunciation? According to the folks at Merriam-Webster, all of the variants that appear in the Collegiate Dictionary, Tenth Edition (except those restricted by a usage or regional label) are widely used in acceptable educated speech. If one particular variant is used more frequently than another, the former will be listed first, but as the editors caution, "The order of variants does not mean that the first is in any way preferable to or more acceptable than the others . . . . In many cases the numerical distribution of variants is equal but one of them, of course, must be printed first."