In U.S. v. Johnson, the Fourth Circuit in an opinion by Judge Wilkinson, joined by Chief Judge Wilkins and Judge Luttig, explained why it is that a sentence within the federal sentencing guidelines carries a presumption of reasonableness.
Judge Wilkinson wrote:
(1) "The first reason that Guidelines sentences are presumptively reasonable under Booker is the legislative and administrative process by which they were created."
(2) "The second reason that Guidelines sentences are presumptively reasonable is that the process described above has led to the incorporation into the Guidelines of the factors Congress identified in 18 U.S.C.A. § 3553(a) as most salient in sentencing determinations."
(3) "the third reason why Guidelines sentences must be treated as presumptively reasonable . . . [is] that such sentences are based on individualized factfinding and this factfinding takes place in a process that invites defendants to raise objections and requires courts to resolve them."
Professor Berman says, even if all that's true, there still can be unreasonableness.