Restarting an instance (not stop-start) doesn't change the hardware you're on, so I don't think this has a physical explanation.
stop/start can possibly put you on another physical - but it all depends on how aws has setup the hypervisors and their instance schedulers.
this, the maint from aws, appears to be security related - but that doesn't mean that aws is not getting folks off of old hardware if they have that desire.
It's more like a system restart with a little downtime managed by them.
You can try a stop-start yourself, but it's not guaranteed to help. And a restart yourself doesn't do anything.