So's the beholder's design...and many, many monsters from 2nd Ed.
Like anything, it can be explained away with decent roleplaying. The wizard who didn't keep up on his wizardry studies gets really good at practicing the same spells, or the fighter finds that he's actually pretty talented at this magic stuff and wonders if he's wasted his life focusing on swordplay.
And I still maintain that neither should be as accomplished as a pure mage, even within their own limited repertoire of spells.
To me, increasing in spellcasting power is not just about repetition and practice, its about gaining broader and deeper understanding of just what it is you do.
I personally have been told by music educators that I have a gift for playing stringed instruments. And at times, its been quite evident. However, I don't put nearly as much into practicing guitar as a professional guitarist. i guarantee you that even though I've been playing guitar since 1985, a pro who knows the same songs I do will be able to bring MUCH more to the performance of any of them than I could...even if he just learned last week what I learned 2 decades ago.
To extend this thought, my Mom is actually a former voice & piano music teacher (she doesn't teach strings). A friend of ours is a professional studio & performing jazz pianist. Despite having decades of experience on him, my Mom can't match some of his skills. During a discussion of the great jazz piece, "Take 5"- a song written in 5/4- while sitting at my Mom's baby grand, he and I discussed a bad cover of the song done in 4/4. That time signature robbed the piece of much of its power. I asserted that if you were going to cover "Take 5" and change the time signature, you should go in the opposite direction, "like playing it in 6/4 I said.
So he did.
"Or 7/4"
So he did...and he did so with every time signature change I suggested.
That's a reflection not just of his mastery of "Take 5," but of all the other music theory he's learned and applied over the course of his career.