there are a few possible scenarios.

1) you already described. perhaps there were 11 people who hit that "check-in button a few seconds before you.

2) doesnt CO connect their ANC passengers through SEA? if so, they can check-in for all flights before you.

3) partner airlines. not sure how that exactly works, but if NW or AS customers flying on the SEA-EWR flight can check-in because they are connecting in SEA, they again, may be able to check in before you.

i always fly out of msy. i am usually the first in line from the iah-xxx or ewr-xxx flight because i can check a few hours earlier. espcially for early AM flights where i am not fighting many other connecting passengers.