Memory by free -b(263307264 about 251MB != 256MB)

[Osamu Aoki <debian@aokiconsulting.com>]

I never bothered memory size until now (<64MB).

How come memory is less than actual size (256MB)???
It looks like 251MB but it is exactly 256*1024*1024-5008*1024

Any one have idea???
--------------- my system info -----------
$ cat /etc/lilo.conf|grep append
#append="mem=256M hdc=ide-scsi apm=on"
append="mem=268435456 hdc=ide-scsi apm=on"

$ free -b
             total       used       free     shared    buffers     cached
Mem:     263307264   45887488  217419776   32391168    4587520   25538560
-/+ buffers/cache:   15761408  247545856
Swap:    271355904          0  271355904

$ free -m
             total       used       free     shared    buffers     cached
Mem:           251         43        207         30          4         24
-/+ buffers/cache:         15        236
Swap:          258          0        258
-- 
~\^o^/~~~ ~\^.^/~~~ ~\^*^/~~~ ~\^_^/~~~ ~\^+^/~~~ ~\^:^/~~~ ~\^v^/~~~ 
+  Osamu Aoki <debian@aokiconsulting.com>, GnuPG-key: 1024D/D5DE453D  +
+  For my debian quick-reference, peek into:                          +
+                               http://www.aokiconsulting.com/quick/  +