Mihawk likely "trained" zoro the same way Raileigh "trained" Luffy.
Make no mistake, Mihawk didnt teach his skills and stuff like that to Zoro he only gave Zoro basic guidance to controll and improve his haki....Dont forget, Zoro already had CoO, CoA and even CoC( if asura itself is that) way before anyone else in the crew, including even Luffy. Its only natural he had a bigger growth from the start of the TS when it comes to haki.