I suspect it’s a legal strategy he concocted with his lawyers: Chemical castration might have a different time period in which it is applied (because longer duration), maybe even starting right after the sentence becomes effective. As the summary here states, the physical version that he opted for himself(!) is not to be applied until a week before the sentence ends, which gives him a chance of a lot of things to happen before, laws to change etc & eventually get out of this without being castrated at all.