Great question! This is one of those AWS design choices that makes total sense once you unpack the reasoning behind it. Let’s break it down:

• Physical Isolation & Fault Domain Separation
  AWS runs multiple physical data center clusters within each region. To stop a single failure event (like a power outage or network glitch) from hitting every customer, they don’t link the same physical cluster to the same AZ name across accounts. For example, your us-east-1a might point to Physical Cluster X, while another user’s us-east-1a maps to Physical Cluster Y. That way, if Cluster X goes down, only accounts tied to it are affected—not every AWS user in the region.

• Load Distribution Across Infrastructure
  If every account used the exact same AZ name-to-physical-cluster mapping, popular AZ names (like the first one, "a") would get swamped with resources and traffic. By randomizing these mappings per account, AWS can spread customer workloads evenly across all available physical clusters, avoiding resource bottlenecks and keeping performance consistent for everyone.

• Enhanced Disaster Recovery Resilience
  This mapping strategy adds an extra layer of robustness. If a major disaster takes out a physical cluster, only a subset of customers (those whose accounts mapped to that cluster via their AZ names) will face downtime. This shrinks the "blast radius" of any single failure, making the overall regional service far more reliable.

A quick pro tip: If you need to align AZs across multiple accounts (for cross-account high-availability setups, for example), use AZ IDs instead of the friendly names. IDs like use1-az1 are consistent across all accounts in a region—they always refer to the same physical cluster, no matter which account you’re working in.

内容的提问来源于stack exchange，提问作者loser8