Not really, it's just the two refer to two different scales, which I'll admit can be sub optimal if you are looking at it as apples and apples, but what you're looking at is apples to apple trees.
I smash you in the elbow with a hammer, this tears the muscle, severs nerves and breaks the joint. A total -70 penalty. . .a "severe" injury.
None of these elements were destroyed, but the penalty is over the threshold where this needs the major versions of muscle, nerve and joint repair spells to fix.
Light/Moderate/Severe refers to the whole of the injury.
Minor/Major/Regenerate refer to fixes to the specific sub system that make up the injury.
An X Muscle repair won't fix that severe injury, because the injury effects nerve, bone and muscle.
I agree that specific critical result is flawed, in that it fails to specify any penalty at all. . .which is kind of irritating, you can be paralyzed from the waist down and drag yourself forward with your hands and stab your foe in the leg. . .it's not a total incapacitation per se, though it might be if the actual penalty is -100 or -150. . .if you have a PC in that situation, and says they drag forward and have at legs with a dagger, what's the penalty?
That aside, the "Severe" nerve injury is definitely -51 or worse, and the word "destroyed" is not used, so you know the minor nerve repair isn't enough, the major is sufficient, and the regenerate will work, but is not mandated.
I find that critical problematic in terms of how you'd play out the remaining combat, short of just saying "you're paralyzed and lay there." but in terms of mapping it out to the right spell, it's not problematic. The correct spell to heal that injury is Major Nerve Repair, minor won't work, you could use Nerve Regeneration, but the lower level MNR will work so that would be overkill use of a 30th level spell.
It does require following the logic chain of:
Severe = worse than -50 so minor won't work
The word "destroyed" is not used, so Major will work
But I don't find that a crazy reach of logic.
I will admit it could be made clearer. (Especially if that critical had a numeric penalty listed).