The phonetics in High Valyrian work very differently to those used in English, so many English names have to by "valyrianized" to fit into the language's phonetic and writing system.
A few more examples: Cersei = Sersi, Jaime = Iēmi
I'd add that this is common in real world languages as well. Prussia never had a king called Frederick - it had several called Friedrich - yet in English we still speak of Frederick the Great. Meanwhile, the Germans call Henry VIII Heinrich VIII. Even today, there's always going to be some argument about romanizations of names from languages that use different writing systems, and English tends to omit the letters that aren't native to it (a recent example would be Pete Buttigieg - it's Buttiġieġ originally, and the dots above the g matter). So High Valyrian isn't unique in this regard at all, or even unusual, in its valyrianization of names and words.