tsearch2 anomoly?

I'm having trouble understanding to_tsvector. (PostreSQL 8.1.9 contrib)

In this first case converting 'gallery2-httpd-conf' makes sense to me
and is exactly what I want. It looks like the entire string is
indexed plus the substrings broken by '-' are indexed.

ossdb=# select to_tsvector('gallery2-httpd-conf');
to_tsvector
---------------------------------------------------------
'conf':4 'httpd':3 'gallery2':2 'gallery2-httpd-conf':1

However, I'd expect the same to happen in the httpd example - but it
does not appear to.

ossdb=# select to_tsvector('httpd-2.2.3-5.src.rpm');
to_tsvector
---------------------------
'httpd-2.2.3-5.src.rpm':1

Why don't I get: 'httpd', 'src', 'rpm', 'httpd-2.2.3-5.src.rpm' ?

Is this a bug or design?

Thank you!
Bob