declare same alias twice, alias values are different, why ?

[cq04cw]

Recently, i have learnd sh(1) manpages, and i have a doubt for an 'alias' command. there's an example below.

> sh
$ set -E
$ echo $-
imsE
$ alias
$ alias aa='cmd_a -a -b'
$ alias
aa='cmd_a -a -b'
this first time is ok
$ alias aa='cmd_a -a -b'
$ alias
aa='cmd_a -a -'
second time to declare the same alias to override the old one, but the option -b is gone, why is it?
$ uname -a
FreeBSD foo.bar.com 8.2-RELEASE FreeBSD 8.2-RELEASE #0: Fri Feb 18 02:24:46 UTC 2011     root@almeida.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  i386
$ ident /bin/sh
/bin/sh:
     $FreeBSD: src/lib/csu/i386-elf/crti.S,v 1.7.22.1.6.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/lib/csu/i386-elf/crtn.S,v 1.6.22.1.6.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/lib/csu/i386-elf/crt1_s.S,v 1.1.2.3.2.2 2011/01/22 17:49:37 kib Exp $
     $FreeBSD: src/lib/csu/common/crtbrand.c,v 1.6.2.1.6.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/alias.c,v 1.25.2.3.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/arith.y,v 1.22.2.3.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/arith_lex.l,v 1.24.10.2.4.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/cd.c,v 1.36.2.5.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/bltin/echo.c,v 1.14.30.1.6.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/error.c,v 1.26.10.3.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/eval.c,v 1.64.2.5.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/exec.c,v 1.34.2.5.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/expand.c,v 1.55.2.10.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/histedit.c,v 1.31.2.6.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/input.c,v 1.25.2.4.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/jobs.c,v 1.72.10.6.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/mail.c,v 1.14.10.3.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/main.c,v 1.31.2.6.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/memalloc.c,v 1.28.2.4.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/miscbltin.c,v 1.41.2.3.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/mystring.c,v 1.13.30.2.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/options.c,v 1.27.2.4.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/output.c,v 1.21.2.3.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/parser.c,v 1.63.2.10.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/redir.c,v 1.27.2.2.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/show.c,v 1.23.10.3.2.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/test/test.c,v 1.54.2.1.6.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/trap.c,v 1.34.2.1.6.1 2010/12/21 17:09:25 kensmith Exp $
     $FreeBSD: src/bin/sh/var.c,v 1.39.2.6.2.1 2010/12/21 17:09:25 kensmith Exp $
$