Sorry for the length of this reply...hope you find it interesting...
I have been hitting this subject quite hard over the last while...for the -400 the figures quoted are a 3% increase in cruise efficiency. "Winglets provide a theoretical straightening of the wing vorticies" My understanding is that winglets are a compromise...the same benifits would come from a re-designed wing (777, 767-400) or by an increased span. The 747-400, A340 & 330 all have large wings and are reduced in design philosophy to the size of airports and there gate design...if the wing is too big it will not fit into the available gates which would make it useless...no sense in savings by the wing if it cant fit into an airport. The 747-400 used winglets to save in cruise so they would not need to grossly increase the span. Each wing is increased in span by 6 ft and then has the addition of the 6 ft winglets, though this is generally for cruise the winglets actually increase the a/c's wingspan for takeoff with a full load of fuel. The wing bends or flexes with a full fuel load and will increase the span from 211.5ft to 213ft, a small but effective increase in wing for t/o.
The 777 wing and I am sure the 767 wing are obviously unique. They are very new and have been completely re-designed. Boeing took the 757 and 767 wing and made more improvements on it for the 777 which is the most efficient wing ever designed. The 777 wing is fatter than most conventional designs and was able to take advantage of stronger materials for its build...which I am sure is where the difference comes from now that I think about it...the 777 and generally boeings a/c philosophy seems to be that they will use the more expensive, modern materials that are stronger and would allow for differing designs...the B 777 has a very hight "taper ratio", chord length from the root to the tip. This was and still is not normally used as it requires a very strong wing, and use of very expensive materials to create a stable wing tip...that wont move and flex beyond an optimum range. (all wings will flex and are supposed to but too much would bend and twist the wing, disruppting airflow.)....so without going on for too long, the 777 and 767 are using a different disign which requires more money to build and a higher a/c cost, I am sure that Airbus is using winglets to get similar results at a lower cost for competition reasons. Also just for interest sake the 777 was designed with an optional folding wing. The a/c was designed initially to fit into gates that were used by the DC-10, L1011 and the like...so to fit the a/c into smaller gate sizes or to allow the company some flexibility in its routes and use of gates the 777 was given this folding design like a carrier a/c...that I know of as of yet none have been ordered with it.