Possibly pushed it back from Spring to an October release? If it was set to come out in March or so, then the A19 Pro would have been the best chip available to go into it, while if it's been pushed back until after the next iPhones launch, then the A20 becomes an option, so why not. As it doesn't get updated for a good 2-3 years between releases there's no real incentive to hold the chip back a generation as it ends up being one of the last devices sold on a platform most cycles anyway.