So I got Sarah Steele's first promotion and I wanted her to be my second companion, behind Subodai. Ratbeard is third. But, when I enter a fight, there is always Subodai and Ratbeard, and not her! So I putted her on firstmate, Subodai second, Ratbeard third. Now she and Ratbeard are coming, and Subodai isn't! Should I put Subodai in third place?
It is supposed to be random, weighted in favor of the companions lined up toward the front. Any time you have random results, weird patterns seem to appear. That said, it seems to be that the #2 person (first after your first mate) doesn't appear as often as #3. Probably statistically not true, but it does seem that way. Mind you, that impression comes after dozens of fights, not the hundreds you would need to spot a genuine statistical anomaly.