The in-game explanation is that he was so studious and didn't talk with that many of his fellow students, so his vocabulary and manner of speaking are more reflective of what he read all the time than what he heard.

So now what I absolutely need is Urianger mispronouncing a word that he'd only read, and never heard anyone actually use in spoken language, because this is a thing that happens with highly educated English speakers! If we don't hit up the dictionary, we can continue to mispronounce it for years. I had a giggle fit when my (very intelligent, highly educated) husband mispronounced "epitome" as "eh-pee-tome" because he'd never actually heard anyone say the word aloud. My personal pitfall was "faux pas" because we have no local French speakers in my corner of the US, so I said "fox paws" for years until someone had to correct me.