Probably because of its preference for the mission for which it does get allocated (as generated by the allocation linear program). Here’s a simple example:

We have 1 pint of vanilla ice cream, and 1 pint of strawberry. We both favor vanilla first, but since my preference (#2) for strawberry is higher than yours (#3), I'll get stuck with my second choice & you'll get your first choice.

• Option 1
  • You: Vanilla (#1)
  • Me: Strawberry (#2)
  • Sum of preferences: 1 + 2 = 3

• Option 2
  • You: Strawberry (#3)
  • Me: Vanilla (#1)
  • Sum of preferences: 3 + 1 = 4

To minimize the “cost” (the sum of the preferences), you get vanilla, I get strawberry.