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Your us-east-1b may be the parents us-east-1c.

The letters are randomised per AWS account so that instances are spread evenly and biases to certain letters don't lead to biases to certain zones.

Huh, that's interesting. Didn't know that, but makes sense.

It's pretty cool. If I recall, they call it "shuffle sharding."

You can check which availability zone is with: aws ec2 describe-availability-zones --region us-east-1