pgbench filler columns

While looking at the compressibility of WAL files generated by pgbench,
which is close to 90%, I first thought its because of the "filler" column
in the accounts table. But a comment in pgbench.c says:

/*
* Note: TPC-B requires at least 100 bytes per row, and the "filler"
* fields in these table declarations were intended to comply with that.
* But because they default to NULLs, they don't actually take any
space.
* We could fix that by giving them non-null default values. However,
that
* would completely break comparability of pgbench results with prior
* versions. Since pgbench has never pretended to be fully TPC-B
* compliant anyway, we stick with the historical behavior.
*/

The comment about them being NULL and hence not taking up any space is
added by commit b7a67c2840f193f in response to this bug report:
http://www.postgresql.org/message-id/200710170810.l9H8A76I080568@wwwmaster.postgresql.org

But I find it otherwise. On my machine, accounts table can only fit 62
tuples in a page with default fillfactor. The following queries show that
filler column is NOT NULL, but set to empty string. I have tested on 8.2,
8.4 and master and they all show the same behavior. So I don't know if that
bug report itself was wrong or if I am reading the comment wrong.

postgres=# select count(*) from pgbench_accounts where filler IS NULL;
count
-------
0
(1 row)

postgres=# select count(*) from pgbench_accounts where filler = '';
count
--------
100000
(1 row)

Thanks,
Pavan

--
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee