That's the "hard to deflect" effect some keyblades have, meaning that you can counterattack most of the enemes attacks or tech them, while with other keyblades you will stumble more often if the enemy is stronger.
if you pick shield or sword and your main dmg input is your keyblade use the olympia while you get divine rose.
EDIT: or at least that is what I understand, I loved Olympia because of its counterattack oportunities, so useful against most of the heartless, like wizards while they are using thunder, it is a
pwner. RIP in peace Oblivion.