Default Access Exclusive Lock on Update?

This may be a newbie question, but according to the 7.4 docs, an ACCESS
EXCLUSIVE lock is only acquired by the ALTER TABLE, DROP TABLE,
REINDEX, CLUSTER, and VACUUM FULL commands.

However, when viewing pg_locks during the execution of a stored
procedure that does not perform any of the above commands, I see that
the table it is working on is locked by ACCESS EXCLUSIVE.

I have also tested that I can not perform a simple SELECT on the locked
table while the SP is executing.

Can anyone provide some insight?

Thanks