No indication is given about the next to fire (at least that was never implied, or explicitly mentioned), it's about the probability. The shaded red disks simply show which chambers might have rounds in, and which might be blank. Rather that enumerate all possible combination of the "6 choose n", I showed the canonical form of "two together" (of which there are 6 cases), "one chamber inbetween" (ditto) ... etc
I tried to make that clear with the phrase "without loss of generality"