I didn’t notice his voice sounding too off, myself.
I wouldn’t be surprised if they deliberately modulated it differently. As I recall, vader’s modulation between Star Wars and the Special Editions (and maybe between the rest of the Original Trilogy, I forget the details) was changed a little.
Perhaps that might be a factor as to why he sounds off as well; Perhaps they used a different reference for how to modulate his voice from what we here usually use as our reference?