Hmmmm...maybe on the dual rating, I've not really checked much lately, used to be pretty common to not be wet-rated, but high temp (what commonly knew as "appliance wire")....
But doesn't address directly the question of whether "wet location" is synonymous w/ "underground" -- I didn't think it was/is and that should still be U rather than W.
OK, I looked at Southwire site and their designations -- I guess probably in conduit the "W" rating is equivalent and the "U" would be only a real difference for direct burial for the extra protection against physical damage (abrasion, nick, etc.).