I think iOS's autocompletion/autocorrection cares only about frequency of use. Both "sylvestris" and "Proginoskes" were automatically added to my iPhone's suggestion dictionary, even though the latter is a proper noun. What amuses me is the separation between iOS's autoco(mpletion|rrection) and its spellchecker. "You use this collection of letters enough that it's probably what you want, but I still don't think it's a real word."