When something is ostensible it appears to be the case but might not be. Research for a role is the ostensible reason for Nat's drinking. But I think it's because he just likes to drink.
Just because something is ostensible doesn't necessarily mean that it's not as it appears to be, only that there's a possibility of another reason. My ostensible reason for calling Brent was to ask about work. The real reason was that I was hoping he might ask me out. Turns out both the ostensible reason and the real reason were true!