because chrono means time (chronological order=ordered in time, chronometer=clock of some sort(i think)) hence chrono trigger, ie time trigger. and they just dont need to call the person chrono, crono is a good name, and naming your son time is kinda stupid (but we know square isnt afraid to use stupid names) and so the answer is probably they just felt like doing one of those fancy name unspellings (like relm=realm, fayth=faith)