Windows ServiceAccount.LocalSystem can access a User session?

Question

I am using Windows Service inside my EXE package.

And I am trying to understand what the difference between these two codes:

First code:

this.serviceProcessInstaller1.Account = System.ServiceProcess.ServiceAccount.LocalSystem;

Second code:

this.serviceProcessInstaller1.Account = System.ServiceProcess.ServiceAccount.LocalSystem;
this.serviceProcessInstaller1.Password = null;
this.serviceProcessInstaller1.Username = null;

I noticed that in the second code, my service can't access a user space, like to get a list of all running processes, but in the first one it is possible. Why is it a such difference? Where I can read more about it?