Airship Guide

[Grimmel]

It's not the SS rank that gave you 2 spoils. It was the Optimal yield.

In previous screenshots where people have only gotten one Iron Voyage Spoil, the Optimal Yield was the same as mine.
If you look at my Screenshot and the one two posts below yours, you can see the optimal yield was the same as mine. The only difference is that theirs was a regular S rating (which I believe is the second best rating), where as mine was a SS rating (the best rating so far). So I do think the rating has something to do with the quantity.

Let me go through the entire log of the previous voyage to see if there were any other conditions noted that could have possibly resulted in the 2 Iron Voyage Spoils being brought back.