why do they say it that way?

[CoriKaru]

Only Utada really knows why she chooses to pronounce certain words in certain ways.

She doesn't pronounce all of her english with a Japanese twang. In fact there's a great amount of english in her songs that is pronounced "correctly" and every once in a while there's some that she pronounces like a Japanese person would pronounce it so there really isn't some divine answer.

With all of the "regular sounding" english that she sings it really shouldn't even be an issue when she does actually Romanizes/Kana-izes it up the little bit that she does.