Probably either the windings or the combination of materials used to make them. Stuart played around with several different "formulations" for strings before settling on this string. I like them.
They last a pretty good amount of time. The set on my NS-4 is the original set. I've reconditioned them once in the past year and just recently they're getting to the place where they're starting to loose their punch and sound a little dead.
I soaked them in denatured alcohol for 24 hours about 6 months ago, then strung them back up. So with light/moderate use they've lasted a year.