The mean or the average of the count in blackjack is zero as there are an equal number of low cards and high cards in any shoe or complete number of decks. The further away the count is from zero, the stronger the tendency that it will move back toward the mean. Professional card counters gain an advantage by wagering larger bets during positive deviations in the count when there is a greater probability that favorable high cards will be dealt as the count returns to its center of zero.
In the financial world, conventional wisdom is to buy when a stock is undervalued and to sell when the price is high. The law of regression to the mean states that probability wise, extreme values are more likely to be followed by less extreme values. Reversion trading is based on buying or selling stocks that are out of line with their “normal” pricing. Traders profit when market prices return to their average just as card counters do when positive counts move back to the mean count of zero.