My first question is: ...
This is not quite the case. There are two situations where the auto-entered calculation will fail to update where a field of type calculation will not:
1) pre-existing records will not update. If you add a new field with an auto-entered calculation, or edit the expression to change the calculation. The data in records created prior to this change will not update automatically. See this thread for a way to get them to update: Updating values in auto-enter calc fields without using Replace Field Contents
2) if your auto-enter calc refers to data in a different record such as a field from a related table, changes to data in that field will not automatically casuse this calculation to update.