I'll reiterate to simplify, because you seem a little slow today:
Top 3 google definitions for master:
- a man who has people working for him, especially servants or slaves.
- a man in charge of an organization or group.
- a skilled practitioner of a particular art or activity.
Zoro works for Luffy and Luffy is in charge of the Mugiwara crew. Definitions 1 and 2 apply.
Definition 3 applies for Mihawk's relationship to Zoro, but I was not using the word in that context and said as such afterwards.
Luffy can give Zoro orders, Mihawk can't.