The rankings were based on your last 3 years results. Zagitova was in her first senior season. In Olympic years, being Olympic champion carries more weight than winning world's. Im sure you know that. Jeez.But you said Osmond was "never considered the best in the world" when objectively she WAS the best in the world.
Zagitova was arguably the skater in the world that year who got the best results (though some might argue Medvedeva was the better skater)... but at the 2018 World Championships, Osmond was the best skater, hands down.