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)= 707200
5*(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