You create the ComboBox at the form level using the ComboBox wizard. These cbo's can certainly reference tables for their Lookup. If you choose to call these tables Lookup Tables, that's fine.
When you defined a lookup for a field at the table level, the form creation used this information to assign a control for that field. The ComboBox that was created on the form was actually a stand alone control that was complete. It no longer needed the field to have a lookup. The field lookup could have then been changed to a TextBox without affecting the form in the least. When you look at a table directly in datasheet mode and there is a Lookup Field, what you see is the lookup value and not what is really in the field. When the Lookup for the field is changed to a TextBox, the same datasheet view of the table reveals what is actually stored in the field.