ISO is best implemented by analogue amplification of the signal before digitization. Otherwise you might as well not bother as digital amplification (i.e. multiplication of the numbers) can be done just as easily in post, and without the risk of blowing highlights.

The Canon majority of Canon bodies that do support fractional ISO stops actually don't have additional hardware to perform analogue amplification and do in fact "fake it" by fiddling the numbers, i.e. ISO640 is actually ISO800 but with the numbers multiplied by 2/3