Calculation of a shared memory

Hi.

I think that a calculation sheet will be made.
However, calculation does not suitable.:-(

It is this.(8.2.5 Default-FreeBSD)
--
max_locks_per_transaction 64
max_connections 40
max_prepared_transactions 5
shared_buffers 28MB
wal_buffers 64kB
max_fsm_relations 1000
max_fsm_pages 179200

inet% ipcs -b -m
Shared Memory:
T           ID          KEY MODE        OWNER    GROUP           SEGSZ
m       917504      5432001 --rw-------    saito    wheel     32571392

40*(400+270*64)=  7072005*(600+270*64)=   89400          28MB= 29360128          64kB=   65536       1000*70=   70000
  179200*6= 1075200
 

Total  :31367464
ipcs-m:32571392     -1203928 (difference is large: why?)

Table 16-2. Configuration parameters affecting PostgreSQL's shared memory usage
http://www.postgresql.org/docs/8.2/static/kernel-resources.html#SYSVIPC
It may be the bug of postgres or whether the document is wrong.... 
or My misapprehension.

Can someone see a near value?

Regards,
Hiroshi Saito