No, this has nothing to do with your stock portfolio.

TLDR version: ratings to actual gains are lower than expected for dodge, parry, and miss.

Slightly longer version: You won't the full 2.13% dodge from items like Valor Medal of the First War due to diminishing returns. In other words, it's like how you need more rating for a stat to get the same 1% as you move up in levels. As an example: let's say you are currently at 15% dodge. In theory you would be at 17.13% dodge if you added the above trinket. In practice you would be somewhere below 17.13% dodge depending on how much dodge rating, defense rating, agility you already have.

Super Long and Wall-O-Numbers version: See the Tankspot Article

Ummm...there is nothing concerning cake or pie here

So...PIE!

That would explain the numbers I was seeing. ("Hmm... these rings are the same except this one has parry rating and this one has dodge rating. And if I try them both on, I get less avoidance from the parry rating ring. Guess I'll be stacking dodge then.") I guess the diminishing returns means they didn't want tanks getting to be nigh invulnerable. I'll have to shelve my plans to don a blue suit with tiny antennae then....

Parry is different because it is our least favored stat. 12 Dodge rating = 1% dodge and 15 Parry rating = 1% parry (before diminishing returns).

At our gear level, though, your diminshing returns are not that signifcant. We are much higher on the scale with armor than we are with Dodge/Parry. Pregor is at a combined 41% which I only got recently after focusing on staminia for so long.

Rating Buster helps a TON so you can see what the true upgrade is. It takes diminishing returns into account.

I started noticing about a 20% drop in dodge value around 200 rating, which makes it roughly as valuable as parry at that point in terms of pure avoidance. Parry, though, gets hit much harder by the diminishing returns, so you can't switch over to parry after getting a certain amount of dodge. Technically defense gives the most avoidance/mitigation value per rating point, but once you reach the crit cap, you're so close to getting hit by diminishing returns, that it does no good to try and stack it after that. Blizz has eliminated any variance in tank itemisation with the expansion, as Stamina is now significantly more valuable than any other stat (at a fairly easy to reach point--i hit them without any raid drops), since it always holds the same value. Gone is the war between avoidance tanks, reasonably balanced tanks and idiots who do nothing but stack stamina. Idiots win! I think that should be the slogan for the Wrath expansion. It's disturbing. Not that I hate the expansion altogether... there's just considerably less thinking involved. Okay, mini-rant over. At least block rating isn't affected, though there's not much of it out there. Fury ftw.

Idiots win!


WOOHOO!

Rating Buster helps a TON so you can see what the true upgrade is. It takes diminishing returns into account.

I use Rating Buster and Pawn to help compare items. Pawn lets you set your own weights, which gives you a quick eyeball to select an item. You do have to set your own weights, though. http://pawnmod.trenchrats.com/index.php might help you set these weights.

For the Bears and Kitties out there, I use the following. It takes into account the 3.0.8 armor bonus changes:
Stat Weightings (tanking):
Stamina: 3.19
Agility: 2.54
Armor: 0.59 (leather) 0.12 (other)
Defense: 1.38
Dodge: 1.36
Expertise: 0.71

Stat Weightings (DPS):
Strength: 2
Agility: 1.565
Haste: 0.765
Crit: 1.025
Hit: 1.133
AP: 1
Armor Pen: 0.922
Expertise: 1.033