The 777 has a more advance wing development within the category of fuel emission. This is where you will see that the 787/747-8 will have wing constructions as on the current 777.
737-200/400/500/600 do not carry winglets for the time being. Usually aircraft carry winglets are there to save fuel, and for your case when you flew back, SW probably have not had that aircraft fitted yet, but most likely further on it will. Hope I answered your question.
Maybe someone else could explain in a bit more detail if they wish.