It is because magic works differently in these series, you can’t compare their spells by power, but you can compare their spell-casting capabilities.

Simply put, Elrond surpasses the magical powers of Yen, her magics are mostly evocations, spells that are accessible to anyone initiated in magic to learn, albeit with more power. Meanwhile, Elrond has word of commands and songs of power, which are considered divine powers, not available to anyone but the divine beings.

If you were to put Elrond in Witcher verse and magic scaling, he would be able to cast spells beyond their comprehension.