This is a mistake from Oda
Zoro is supposed to always wear his sword on the right and that's intentionally wrong
There's an old sbs asking why Zoro wears his swords on the wrong side and Oda says:
"Zoro has bad posture and he sort of leans to the left so if he wore his swords on the correct side, he would end up leaning even further off. So Zoro wears his swords on the wrong side to balance off his bad posture"