According to the AWS doc:
When you stop a running instance ... The instance is migrated to a new underlying host computer when it's started.
Here is also slightly different statement from the same doc:
When you start your instance, it enters the pending state, and in most cases, we move the instance to a new host computer. (Your instance may stay on the same host computer if there are no problems with the host computer.)
So what is the rationale behind the need to move instances to a new host when they are restarted?
The only reason this question has emerged is that I found a question in an AWS Certification Exam sample asking whether migration to new host happens, and currently see no way how one would be able to guess the correct answer.