A bit confused about some gear. Unexpected loss of Defense

I have been using the belt of the guardian socketed with 2x 12 strength. http://www.wowhead.com/?item=30034

Yesterday, I got http://www.wowhead.com/?item=36952 from The Oculus.

When I tried to mess around with it this morning to see what my stats looked like after the swap I was amazed to find out that I would actually lose some defense skill upgrading form the belt of guardian to the new blue belt.

Stats with belt of the guardian:

Defense 535
Dodge 16.86%
Parry 15.31%
Block 10.40%

Stats with Girdle of Obscuring

Defense 533
Dodge 16.79%
Parry 15.86%
Block 10.32

0 resilience with both.

Now what confuses me is, how can an upgrade of 10 defense rating, cause me to lose 2 defense? I've heard about diminishing returns on dodge, parry etc, but I have no clue what this is about really.

Could anyone help me out here? I hate being confused, that's like getting dazed in-game, makes my blood boil

Screenie of me wearing blue belt, with stats shown: http://img185.imageshack.us/img185/3681/wowscrnshot120108082457el7.jpg

Screenie of me wearing belt of the guardian, with stats shown: http://img185.imageshack.us/img185/6582/wowscrnshot120108082450ti4.jpg

Thanks in advance guys, hope you can help me out.

Cheers,

Haldhir, Aerie Peak

Hard to say without an armory link but my bet is that you are using the defense metagem and switching out those red gems is deactivating it
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Sounds like the meta gem is the culprite. Try using your belt slot for red gems if you have none on gear. I am fond of the champion gems (str/def) for my red slots.
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Majiben
If we wanted Ret to be a joke, we would have just left the tree as it was, and maybe kicked it a little for good measure. -GC

Either way, slap a buckle in the belt. Gives you one gem-slot that totally ignores socket-bonus, and it counts toward activating your meta-gem :)
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Kieras <- armory

Ah my apologies, I totally overlooked the meta-gem, when I even had to switch out solid stars of elune for the strenght gems to get it to work in the first place.

I blame the early morning

Sorry guys