[mike-cardwell]

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.

[chrishynes]

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

[ciceryadam]

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

[thrtythreeforty]

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