You dont understand how any pickup could increase sustain?? It's possible because factors like number of turns, size of wire, and magnet can all have an effect on highs, mids, and lows, which in turn is directly influential onto how long a pickup sustain. If one pickup is wound very vintagey and very underwound, chances are it wont have tons of sustain. This isnt to say that an overly overwound pickup will necesarily be dripping with sustain, but many factors go into determining how a pickup will sound, and one of this factor is how much sustain it has.