[HardyLeung]

The USAMO winner list is here:

http://www.maa.org/sites/default/files/USAMO_Winners_2016_1....

Apparently the final team was not chosen strictly from the top 12 (Michael Kural and Ashwin Sah were Honorable Mentions).

[nhaliday]

There are two more tests used for team selection after the USAMO (during the training camp).

[Bahamut]

Could be that they couldn't attend or some other reason.