I read "Each AWS Account has independently mapped AZs, which can vary between different accounts" in a book. As far as I know , each region has its well-defined AZs. So how does it come that the AZs vary from a user to another ?
Availability Zone A for my AWS account may not be the same as Availability Zone A for your account. The AZ mappings are created when your account is generated.
The idea is to distribute customer workloads within a region. Most customers pick AZs A, B, or C when launching instances (human nature). If everyone had the same AZ A, workloads would be hugely unbalanced in a region, and that AZ would be a failure point for a lot of customers.