I don't know if the fluff explicitly states this, but I picture different Lizardmen communities coming together when necessary. I mean, why not? They all serve the will of the Old Ones. And Slann die sometimes, or team up with other Slann.
I own two Slann. One is a relic-priest (Lord Kroak's model, but not Lord Kroak) and the other is a living Slann. My personal fluff is that the living Slann is from a city that, Brigadoon-like, was lost in time during the Great Catacylsm. The dead Slann was one of his close confederates in ancient days. When the dead Slann's city was destroyed, he directed his surviving servants - mostly the skinks - to carry him to the location of his old friend's temporally displaced city. There, he worked a powerful spell to re-anchor his old friend's city in space and time. My army represents some ancient and newly recovered Lizardmen, some modern survivors. This also explains why I own two Slann (and might even field them both, if I ever get to play a big enough game).