Over lower altitudes, stall speed does not vary with altitude. This is because at these lower altitudes, Mach number at Vs is less than 0.4M, too low for compressibility effects to be present. At higher altitudes the CL max (max coefficient of lift) decreases due to compressibility. Thats because the airmass ahead of the wing leading edge must deflect through a greater angle and loses kinetic energy, seperation occurs at a lower angle of attack.
If the CLmax decreases, the IAS must increase for lift to be maintained. Thats why you see the lower airspeed band creep up as you increase altitude.