git describe not matching git log

It seems "git describe" is not matching "git log" as detailed in the help, in some cases. From git describe --help

  [torvalds@g5 git]$ git describe parent
  v1.0.4-14-g2414721

  The number of additional commits is the number of commits which would
  be displayed by "git log v1.0.4..parent".

GOOD

  $ git clone git://github.com/antirez/redis.git

  $ cd redis

  $ git describe unstable
  with-deprecated-diskstore-1050-g7383c3b

  $ git log --oneline with-deprecated-diskstore..unstable | wc
     1050   11779   78709

BAD

  $ git clone git://github.com/git/git.git

  $ cd git

  $ git describe master
  v1.8.0.1-264-g226dcb5

  $ git log --oneline v1.8.0.1..master | wc
      260    1650   14154