Theres always the story that Arnold did not prepare for the O properly and only trained 4-6 weeks.
He'd been training for Conan for a year, decided to do the O the year before (79) already, when he interviewed Zane (and Zane told him the only win more satisfying than his 3rd Mr O win, was beating Arnold in 68!).
He started prepping for the 80 O then, and telling people he was training for Conan and doing the Olympia judging. Rocked up in Australia last moment, obviously the Weiders said no problem, compete.
He knew Conan would only start filming end of 80/early 81 in Spain. He even got George Butler to come out to Australia to do photography and video way ahead of time.