At 11th, plate mail's going to give a +13 to AC. Even a hide-wearing elven archery ranger isn't matching that.
He's not supposed to. It's Plate.
In the PHB, it was +11 to AC (+3 magic, +8 Plate), not +13 and only accessible to the Paladin without a feat.
The elven archery ranger in Hide with +3 Hide, +5 Dex, +3 Magic = +11.
At level 1, it was Ranger AC 17, Paladin AC 18.
At level 11, it was Ranger AC 26, Paladin AC 26.
The monsters are +10 to hit, the Ranger is +9 defense and the Paladin is +8 defense in 10 levels.
Do you really think that someone in Hide should be improve in AC compared to someone in plate?
But, the real problem is not level 11. It's mid-Paragon where this becomes real obvious.
At level 15, it was Ranger AC 29, Paladin AC 27. The Ranger keeps climbing. The Paladin keeps falling behind.
14 levels later, the monsters are +14 to hit, the Ranger is +12 defense and the Paladin is +10 defense. What's wrong with this picture?
That was the problem that the heavy armor masterwork math fixes handled in AV and PHBII. With the fix:
14 levels later, the monsters are +14 to hit, the Ranger is +12 defense and the Paladin is +12 defense.
There will always be a slight descrepancy between light and heavy armors because they use two different equations.
Light armor has a +4 boost due to level ability score increases, a +1 boost to Epic ability score gain, and a +2 boost to masterwork = +7.
Heavy armor has a +6 boost to masterwork = +6.
Light armor wins out in the long run anyway.
You are focusing on one level and saying "Heh, Plate gets +13 here". Err, so. Hide gets +11 and Hide started one level lower. Plate gains a whopping 1 AC for levels 11, 12, and 13 (assuming the PC gets magic armor at level 11).
That's not as bad as Hide gaining 1 AC on Plate at levels 8, 9, 10, 11, 12, and 13; and gaining 2 AC on Plate at levels 14 and 15. That's a lot more unbalanced.
That's what was fixed.