Just look at this photo of our garden and the unusual disparity in size between the brussels sprout plants. These three, and these three only, right next to each other, are easily twice the size of all the others.
We started around 30 plants, all from the same packet, all in the same conditions during all phases, but something different happened right here in the garden. We aren't the greatest at record keeping yet, but I doubt that it would reveal anything. There are so many variables, and you have to be observant all the time, not knowing what event or pattern will cause what effect. Obviously, I'd like to replicate whatever happened here, but for now, I'm just scratching my head.