By the time (and if) Surrey becomes a center of its own, Abbotsford will probably have developed to the point that it would not join with Surrey, but rather develop towards becoming a third center, as the distance between Surrey and Abbotsford is still quite considerable.
In regards to Aldergrove, I am not certain why the gas is cheaper there than in the rest of Metro Vancouver, but it IS (without a doubt) a part of the Township of Langley and Metro Vancouver. After all, the Greater Vancouver Zoo is located there