The games sent are done in random fashion. Even though a game has "Available Now" beside it, that doesn't mean it is in stock from your primary shipping center. Most likely, Portal 2 was available from your primary center, and that's why it was shipped first.

Sometimes you'll get games from a non-primary facility, but if you've got games in your top 5 that are able to be sent from your primary center, you'll most likely get those first.