Yeah I think you are misunderstanding a very cool feature our Dev team implemented here:
spell progression ≠ caster level ≠ spell penetration caster level.
I'll explain it with an example that will make it more clear. Let's say we have Quinn the Palemaster (10 wizard / 10 palemaster), and Ana the wizard (20 wizard)
Ana will have 20 spell progression / 20 caster level / 20 spell penetration caster level.
Quinn will have 20 spell progression / 15 caster level / 10 spell penetration caster level.
How does that affect them? Let's take it to the numbers:
They both have 20 spell progression, so they both have access to the same amount of tier 9 spells as long as they have the same intelligence.
Ana has 20 caster level and Quinn has 15. There are many things that are affected by caster level:
Spell duration: Buffs cast by Ana will last longer, especially the ones that work with turns/level mechanic. For example, if Ana casts Bull's Strength, it will last 20 turns. If Quinn casts Bull's Strength, it will last 15 turns. It may seem minor, but Ana's buffs will last until her next available rest, while Quinn's won't, and that can be a hassle.
Spell Damage: Many damaging spells use caster level to determine damage because they use the caster level as a multiplier. To name a classic, if Ana casts Isaac's Greater Missile Storm it will do 20d4+1 damage (between 40-100 damage). If Quinn casts that same spell it will do 15d4+1 (between 30-75 damage).
Dispelling: The buff dispel mechanic takes into account spell level. Which means if you have lower caster level, your buffs are easier to dispel. And if you have higher caster level, your dispelling is more effective. The formula is 1d20 + dispel's caster level vs. 12 + creator's effective level. It's confusing, so let's make an example. Imagine Ana and Quinn start fighting each other and they both cast Mordekainen's disjunction. Ana's dispelling will be 1d20 + 20 vs DC27. That means Ana has 65% chance to dispel Quinn's buffs. On the other hand, Quinn's dispelling will be 1d20 + 15 vs DC 32. That means Quinn only has a 20% chance to dispel Ana's buffs. This is a BIG deal. And is not only important in PvP, most high level dungeons in this server have monsters that can dispel buffs.
And there is also spell penetration caster level. It's the ability to pierce a target's spell resistance. The formula is d20 + spell penetration caster level + spell penetration vs. Spell Resistance. In an example, let's say Janos the evil necromancer (Cleric 20) casts Spell Resistance spell on himself. And both Ana and Quinn try to fling a fireball at him. Ana's fireball is d20 + 20 vs DC 32. That means Ana has a 40% chance to land the fireball on evil Janos. On the other hand Quinn has d20 + 10 vs DC 32, which means his fireballs have a 0% chance of landing (I am not sure if a natural 20 is an autosuccess here, but even if it was, it only raises the success rate to 5%).
As you can see, Ana is clearly the better spellcaster. Quinn gets some tankiness in return, but in powerbuilding terms alone, they are not worth it, because at the end of the day, Pale Masters are spellcasters, not tanks. It's the equivalent of offering a paladin more spells per level, at the cost of losing AC and AB. Sure, it's nice to get more spells, but not at the cost of being worse at their main role.
I am not saying Pale Masters are a weak class, they are not. They are still spellcasters, and as such, they are stronger than most melee classes, but that's just how DnD works. But they are weaker than their base class that requires no application, so I see no reason to take their tier 9 spells away.