I'm guessing it's different density of the smoke clouds. Even if they rise consistently over the burning area, they could still form patterns of higher and lower density as they move. Think patterns in smoke from a cig or candle as it rises and moves - curls and gaps forming in it. Not sure what that's called or caused by in fluid dynamics but I assume that happens in the smoke clouds as they travel south. So when we get lower density bit over us - numbers go down, hgher density - they go up.