In the Auto-Enter tab, right below the Calculated Value 'specify' is a checkbox 'Do not replace existing value if any'. This is default checked. When it is checked, the value will not update when a value changes in fields it references. If you uncheck this box then, any time the value changes, it will update.
Also, be careful with what fields you use in your calculation. Your example expressions should all work--so this is a caution for future use of this method.
If an auto-entered calculation refers to fields from the same record and table, it will update as LaRetta describes. Include a field from a related table, use getNthRecord to reference a field in a different record or refer to a field with global storage and then changes to these fields will not automatically update your auto-entered calculation and you then have to develop alternatives.