\df+ and pg_proc query

I assume that there is in fact a difference between doing:

\df+ <func_name>

and:

select oid,proname,prosrc from pg_proc where proname='func_name';

because I have a plpgsql function for a trigger, which I can see with the second method (the select...) but not with the first method (the \df+ ). Is it not possible to have the \df actually show up all functions?

TIA,
thalis