Shared memory usage in PostgreSQL 9.1

For a PostgreSQL 9.1.1 instance,
I have used the following postgresql.conf settings:

max_connections = 100
shared_buffers = 400MB
wal_buffers = 16MB

All the other parameters have been left as default values.

When I startup the instance, I get an error message
saying that the shared memory does not suffice
and 451837952 Bytes would be used.

However, this is not what I expect when calculating
the needs according to the documentation, Table 17-2 at
http://www.postgresql.org/docs/current/static/kernel-resources.html

According to that table the usage would be:
Connections: 1908000 Bytes
Autovac workers: 57240 Bytes
Prepared transactions: 0 Bytes
Shared disk buffers: 400MB
WAL buffers: 16MB
Fixed space: 788480 Bytes
Sum: 435145336

This is about 16MB less than what is really requested.

How can this substantial discrepancy be explained?

For PostgreSQL 9.1, some important item must be missing
in Table 17-2, or some values are wrong.

-- Christoph