It makes sense when you consider that it's an artistic choice and not one of necessity. He presumably likes the very overtly autotuned sound for one reason or another.

If you only autotune a little bit, it's quite difficult to impossible to tell that it's been used by ear alone. In order to get his signature autotune sound, you need to make it shift the input by a significant amount, so to get the correct note on the output that means you need to sing intentionally high or low of the intended output by some amount (you can probably change the tone by varying how far off you sing from the intended output).