What are the differences between availability zones for Amazon EC2 in US east?
Availability zone identifiers (us-east-1a, us-east-1b, etc.) are different for every account. So us-east-1e may not be the same as us-east-1e for someone else.
I think part of the reason for this is to avoid situations just like this: certain AZ's getting bad (or good) reputations and having people flock away (or toward) certain AZ's.