There are three factors in play:
All the PHB subclasses, minus a few cleric and wizard, were kept from 14 to 24.
Imported subs from Everything books had to be popular and played.
The subs had to fill a quadrant of options.
That put some classes in different design places then others. The classes that had only two options in the PHB have more design room than the ones with 3/4 choices filled. The cleric and wizard having to pair down options meant they ended up with very similar pairs (attack, protect, deceive, reveal). I also think they tried not to reuse the same themes if possible, which is why glamour (fey) isn't opposed by whisper (shadow) because fey/shadow is a ranger pairing. Obviously, they couldn't do that for every set.