My opinion is that since the Japanese have no alphabet per se that they Americanized the name from the Japanese enunciation of Toi-oh-dah (Toyoda) To Toyota which sounds more American.

I think another example perhaps though not a good one would be the multitude of spellings for Gaddafi,Khaddafi,Qadaffi although for most part we do pronounce it the same way.

I think perhaps a more interesting question would be is why did Datsun which was quite successful under that name suddenly change their name to Nissan