One answer: in general, the "jamband" community actively encourages recording, and you'll usually see several "tapers" at a show, even for small, local acts. Recordings are easily available, via channels like archive.org. As such, it's easy to listen to and compare dozens or even hundreds of individual performances from one band, without actually "going every night".
All of which is beside the point that the term "jamband" is vastly misunderstood and often misused to incorrectly specify a very narrow, pre-conceived notion. In my view, it really encompasses a very broad mix of styles and genres. The one common element is exactly that which started this string of thought - the desire to make each performance unique and in the moment. In that regard, it's usually the case that setlists contain different material and different ordering each night, and there is a healthy amount of room allowed for on-the-spot improvisation.
/rick
Hundreds of different set lists, different licks, different moves? It might seem diferent but I still don't buy it.
And that still wouldn't cover every night.