Well, why else would somebody bother making a new translation of an already-translated work unless they thought that existing translations were flawed?
I'm not just talking about acknowledging improvements; I read one introduction where he spent the majority of the time more or less calling a previous translator an idiot.
But another reason I could see to re-translate something besides previous translations being flawed is them being old. If a particular obscure work only has on English translation, and it's 100 years old, it seems pretty reasonable to want to translate it into more modern English.
