Well, the thing is threefold:
First, Orland(ea)u is a swordsman of legend. He's essentially one of the best swordsmen in all Ivalice, period. The only equal in skill died already, and that was Balbanes/Barbaneth, father of Dycedarg, Zalba(a)g and Ramza. To represent that, they made him a bit more powerful than the rest.
Second, Orland(ea)u has the second best stats and stat progression of all characters. The character with the best increase in stats, believe it or not, is Reis. Orland(ea)u's benefit, however, is that while Reis is a bit of a generalist (needs good Magic for her breath weapon attacks, good Physical for her unarmed damage given her lack of weapon proficiency and automatic Brawler/Martial Arts feature, and good HP for her lack of armor proficiency), Orland(ea)u's stats progress just where he needs them. For example: look at the Physical Attack stat of Orland(ea)u without the Bracer equipped; his should be close to 15, while Ramza and Agrias and Meliadoul and Beowulf quite probably have at most 12. Not only his damage with weapons is high, his sword skills are also powerful (since they are based specifically off his Physical Attack stat).
Third, it's how the system works. Sword skills are based on this simple system: Physical Attack times the sum of the weapon attack power plus the power of the skill. Let's make simple math: Agrias might have mostly 12 Physical, while Orland(ea)u has at least 15 Physical (and that's without Bracer!). Let's say both are using the Excalibur, which has a base attack power of 21, and both use the Stasis Sword/Judgment Blade sword skill, which has a base attack power of 2. While Agrias would deal 276 (12 x [21+2]) maximum points of damage, Orland(ea)u would do 345 (15 x [21+2]) maximum points of damage. The difference in damage is of 69 points of extra damage; of course, that's basically what you get if you multiply 23 (the sum of the Excalibur weapon attack power plus the base power of Stasis Sword/Judgment Blade) by 3 (the difference between Orland[ea]u's Physical Attack stat minus Agrias' Physical Attack stat). However...Orland(ea)u has a Bracer right from the beginning, while Agrias may have another weapon; give Agrias a Bracer and her sword skills should improve, but not to the degree of Orland(ea)u's.
Fourth, and this is mostly an assumption, is that Orland(ea)u's sword skills are 50% stronger than that of a character using the same ability. You'll notice this because, using the same weapon and based on the earlier calculations, Orland(ea)u shouldn't do that much damage. Now, increase that 345 damage an Orland(ea)u without Bracer using Stasis Sword/Judgment Blade with an Excalibur equipped by 50%; the end result would be somewhere around 517 points of damage, which is usually what Orland(ea)u pulls off when on the class. Notice that if Agrias dealt twice the amount of damage that she would otherwise deal, the damage would be around 552 points of damage, which would require her Physical Attack stat to be somewhere around 24 (something naturally impossible unless you tweak the character for that). The difference between the amount of damage Agrias with 24 PA would do against Orland(ea)u's 15 PA and 50% swordskill bonus would be of 35, which would be roughly 1.5 less PA than Agrias would require to have. It's certainly less than Orland(ea)u dealing damage with a Bracer equipped (18 PA, which would mean his damage would actually be somewhere around 414 points of damage), but since damage may be reduced by the Zodiac compatibility factor, you may often see Agrias dealing little damage and Orland(ea)u dealing lots.
So, most of what you can do is give Agrias or Meliadoul a strong Knight Sword and Bracer, while leave Orland(ea)u without a Bracer AND with a weaker weapon. That's enough to "nerf" Orland(ea)u a bit, but he'll still be insanely strong (specially because his PA growth stat is obscenely high).
TL;DR - Orland(ea)u is just THAT strong, but look more towards his PA rather than his OMGWTFBBQ awesomeness. That's a big key-.
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