Actually in Super/System 2 he does qualify the statement by saying it has to do with table image.

Also, there is a mathematical concept known as "regression towards the mean" that explains why sometimes good cards (or bad cards for that matter) come in "streaks". Try flipping a coin about 100 times, you will see that now and then you may get heads several times in a row, but in the end you'll be about 50/50. It works itself out in the long run.