With Dex in tumble. I went dinking with it a little and found it odd regardless of 27 - 29 I cap at 31. And I think my parry/2weapon is at a +4 or +5 I have to mess with gear again to see what the natural bump is. but I believe you are right it should be a Plus four even so at ac 29 it should at least be AC 33. I'm about to head in after a few posts and dink with it and see maybe its due to the cat's grace or barkskin spells I used. It just seem odd when I removed two weapon fighting my base before applying was 29 then when removed it was reduce to 27. It may be too something stalking and removing for the greater buff but I got to knick pick and pay attention to which Ac's ar changing.
Remember that tumble only counts the raw 'ranks' in the skill, not the total (Ranks+Dexmod). Maybe if you list your AC gear and feats that are associated with your AC, we can get a better understanding.
In regards to your parry skills, it utilizes your total skill (ranks+dexmod). Therefore, given your current skill in parry here should be your AC bonus (depending on if you have the below feats taken on your character):
Parry (15) = +3 AC
Parry (15) w/ two weapon defense = +4 AC
Parry (15) w/ improved two weapon defense = +5 AC
This should be easily determined by equipping and unequipping your weapons while monitoring your AC.
EDIT: Here is a link to a thread that discusses it further:
http://www.nwnravenloft.com/forum/index.php?topic=25379.0