Why aren't all of my accounts showing up in the /users/{ID}/associated route?

Question

Every so often, some of the associated accounts for a given user are missing from the response for the /users/{ID}/associated route.

For example: I have an account on Gaming.SE that isn't showing up here:

http://stackauth.com/1.1/users/1825c09e-90bf-44b5-9923-df87662a9082/associated