You say "you might just as well say it reflects their special training". Except it is not training.
You can spend all your life training for certamen and still not be better than a Tremre who has just studied half the time you did. (and with the cost of art and abilities he doesn't even need that).
Let's say you spend time to have 5 in every ability used that's penetration and parma, so 150xp.
You also spend 55xp in 4 arts, which are your "good" ones. As an attacker, you will have a choice between 2 of those 4. But you will have a random chance for the 2 others.
You spend 150+220 xp for this.
The Tremere has spend 15xp *2 in abilities, for a meager 2 and 15x15 xp for 5 in every art.
He is well more suited to crush you as attacker or defender because he can hit one of your 0's (we expect that you manage to get one of your good one for the other !) and still match your good one.
You have a 3 difference in ability which is not enough for his +5 advantage but well.
But that was a almost gauntleted mage. Now if we repeat this operation for your 4 arts at 20, with 4x210 xp, and your 8 in 2 abilities, for 360 xp, that means 360+840 = 1200xp.
The tremere has raised only 4 arts to 15, and every other at 5. That cost him 11x15+4x120 = 480+165 = 645xp.
He spends the same 360xp you did because why not.
He still has spend less than you, and we expact that you hit him with one of your good art, for a magnificient 20, against his meager 5x2
But he hits you with one of his good art, for a 15 against 0..
Now you insert politics and intrigue, the Tremere knows your arts, provoked you for a long time, preparing for you to come duel him, or even fed you false informations about his arts, goals, etc. and one of his 15 is one of the 20 you thought he didn't have. Suddenly your advantage is 20 vs 2x15 (-1!) and still, you are at 15 vs 0. Let's not even speak if your 20 is against his 15 and he is at 30 vs 0 because that would be a landslide victory, if he strikes first.
In that example (which is purely theoretical) he spends less xp, and in the best case for you, he wins after some effort ; in the best case for him, he wins and crushes you with total humiliation.
And if he has vis to match (i'm not even sure any more if vis is allowed during a certamen honestly), that's a bigger winning.
The nature of the arts & abilites cost render his x2 advantage more advantageous the more he goes up.
Iirc the attacker choose the technique, so as the defender, he would be even more advantaged. (He just put 10 in every technique, and 2 forms at 20 for example, which would then cost 5x55+8x15 (not mandatory but to open an apprentice it's fine)+2x210 = 815 or 695 (+360 if we keep that number of 8 in two abilities)
And then it's a 20 vs 2x10 and 20 vs 0.
I personaly don't care much for honor but honor is a tool, and I see Tremere magi to use the honor of their enemies to crush them: either they start unwinnable certamens, or they are shamed in front of their allies and potential supports ("you would help this man? who didn't even stand for himself? and he asks that you fight his battle? sure do, but do not be surprised when he refuses to help YOU since you already know he wont help himself.").
(Of course as there is a lot of randomness in the game, it's not a sure thing. But that's a question of statistics, botch chances, etc.)