The blow off pressures for a fixed width vary. Flow rate of a single waste stream varies over a day.
Faults distorting the railway wheels occur at random.
Even though the chemical composition may be kept reasonably constant, mechanical properties still vary. Coupons from the same panel vary randomly. Tensile strengths for the same walls vary. RDD varies along the pavement and the locations are chosen at random. Blow off pressures, Granite panels, Shotcrete strength.