Why is there such a HUGE discrepancy with class masteries between classes?

Without exception, all NB class masteries are beyond bad for PvP.

1. Nocturnal Inspiration
Upgrades rank 2 of Hemorrhage to have a chance to generate 2 Ultimate. This effect can occur once every 0.3 seconds and the chance is equal to your Weapon Critical chance.

When exactly are you supposed to benefit from this? You don't have any good HoTs on NB to reliably proc it through heals. You have 0 pressure and not a single good DoT to say that at least you have some ticks on a target. You don't even run status effects like burning or poisoned since your damage is so awful you have to rely on shock enchants for minor vulnerability procs. Your DPS is virtually 0 aside from the ocassional spammable and ult drop combo. Even most merciless resolve casts are not to deal damage, but to roleplay a poor man DK's flame lash and hope that you somehow crit hard enough to intimidate the opponent into not parsing on your for 1 second.

2. An Eye for Exploitation
Increases your Weapon and Spell Damage by up to 2000, based on your target's missing Health. Reduces your damage taken by up to 20%, based on your attacker's missing Health. Both effects are halved against targets with Battle Spirit.

Up to 2000 / up to 20% refers to a target being at 0% HP which is also halved in PvP. At most, you'll start benefiting from this passive when the opponent is around the 50% HP mark after you drop your ultimate and get lucky enough to land a crit, in which case you get a 500 WD/SD buff and a 10% damage reduction that is only active while the opponent is actively healing and not damaging you. Let's disregard the fact that any competent player will dodge your merciless resolve shot immediately after you ult, and the window for tge buff is in most cases wasted. We can just compare it to any anothe class:
- Templar, for which Bright Harbinger provides 600 WD / SD at all times, also affects heals AND provides some group utility with a unique 300 WD / SD buff
- Wardens get 1600 WD / SD from class masteries which is not halved in PvP.
- Sorcs get an Exploitation upgrade AND roughly 20%+ WD/SD from Font of Power.

3. Above and Beyond
Increases your Critical Damage and Healing by 25%. This effect is reduced to 5% against targets with Battle Spirit. Increases your maximum Critical Damage and Healing by 30%.

5% crit damage is worse than a normal passive. With a 40% crit chance, this mastery translates to 5*40/100 for a grand total of 2% damage increase. The crit cap is completely useless if you don't have a group next to you to provide every crit buff that exists in the game. At least a warden can get Major Brittle which is NOT changed to minor brittle in PvP ( Which would still be effectively double than what NB gets since that's 10% ).

4. Cutthroat's Focus
Activating a Nightblade ability while bracing causes you to dodge attacks for 0.3 seconds. Whenever you dodge an attack, your attacker's psyche suffers, increasing their damage taken by 5% for 5 seconds, increasing to 20 seconds against monsters.

The dodge sounds cool on paper, but is incredibly unreliable and it's more of a lucky accident than something you can do on purpose. 5% extra damage and some unreliable mitigation for a whole class mastery feels like a joke.

- A sorc gets a shield for 0.5s ( not 0.3 ), which doesnt care whether an ability is dodgeable or not and a 6% weapon / spell damage buff for the WHOLE group ( which, again, also affects healing )
- Arcanist gets a reliable 15% damage done
- DKs get 10% but ALSO 10% heals and shields.

As for dodging:
- DK will combo with Inferno, Soul of Flame and Leap, which are undodgeable
- Templar's jabs, PoTL, Crescent Sweep combo is undodgeable.
- Necro's Blastbone, Coloss, Corpsebuster proc is undodgeable.
- Sorc's Haunting Curse into streak is undodgeable.
- Arcanists mostly spam status effects on you, so you also eat damage while dodging and burn stamina for nothing. Tentacles are also undodgeable.
- Warden's Shalks into contingency DB or northern storm is undodgeable.

At least I can dodge another NBs incap or follow up merciless by mistake so he can feel even more miserable about the class.

5. Share the Spoils
Secrets and coin are tools as sharp as the daggers you wield. Upgrades rank 2 of Transfer to grant group members 250 Magicka and Stamina, 2 Ultimate, and doubles the Ultimate you gain when it activates.

Yet another Warden Passive from wish. 2 ultimate per 4 seconds ( or 1 every 2s ) is worse than Minor Heroism ( which is 1 every 1.5s ). Meanwhile warden provides the exact same amount of sustain ( 125 every 2s ) but also gives MAJOR HEROISM.

Spoke with quite a fair amount of very strong NB players the past few days, out of curiosity to see how it goes for them as well. When I ask what they run on NB, the universal answer is "I Don't. I play [insert another class] instead." ( Mostly DK ).