Eh, it varies, they're typically relative in strength but there are cases where Zoro's noticeably stronger. Right now is one of those cases
But you're right that they're usually relative. It doesn't really matter, though, since the Zoro agenda isn't just about him being stronger than Sanji, it's about him competing against other swordsmen, since that's what his actual dream is. As long as he keeps up his streak of never losing and beats Mihawk in the future, as well as permanently hardening Wado Ichimonji, the Zoro agenda is fine.