"IS NOT NULL" != "NOT NULL"

Just a question that's stems from curiosity, but:

db=> SELECT COUNT(*) FROM pkg_hosts WHERE timestamp_col NOT NULL;
 count
-------
     0
(1 row)


db=> SELECT COUNT(*) FROM pkg_hosts WHERE timestamp_col IS NOT NULL;
 count
-------
   1242
(1 row)

Why aren't those the same?  Seems like the IS would be an extra word
that's not necessarily needed.  ::shrug::  -sc

--
Sean Chittenden