Trying to come up with a hypothesis why he might have grain in his cargo hold:
- Could have been a bulletin board mission. They often ask for foodstuffs of different sorts, including grain.
- If it was a ship with a very large hold it might just have been soon after he bought it. Lacking the money to fill the hold with expensive stuff grain actually gives a very good return on investment, in some cases 400% or so. Of course it's 400% of very little, but that's still better than 10% of still very little. You see, as a pirate it pays understanding traders to avoid unfortunate incidents.
But seriously, don't you pirates carry a cargo scanner? If you dropped me out of supercruise in my Asp or Hauler I wouldn't have 12t of cargo to give you since these are my exploration ships and I simply don't carry any cargo. You wouldn't believe me and shoot me up and just because you don't carry a cargo scanner? Shame on you, sir!