Taken from the Powerball FAQ page:
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WHY HAS THE CASH JACKPOT GONE DOWN?
Actually, it is the annuity jackpot that has gone UP. Most people assume that we start with the annuity, but we actually start with the cash jackpot. In the Powerball game, thirty cents of every dollar sold goes into the cash jackpot pool. The difference between the cash jackpot and the annuity jackpot depends on how much we can make in interest earnings. The more interest that we can make, the higher the annuity jackpot. In times of high interest rates, the annuity prize will go UP and the difference between the two jackpot amounts will increase. It is not that we have reduced the cash; but only that we have been able to increase the annuity prize. All of the interest earned is paid to the winner for the annuity.
Interest rates are slowly rising, which does reduce the cash percentage, but a change in the payment method of the Powerball annuity prize is the big reason for the increased interest earnings to build the annuity prize. The annuity is now paid out as a graduated annuity. Each payment is 4% higher than the previous year's payment to help keep up with inflation. The annuity prize used to be paid out in equal payments. Persons who elect to take the annuity prize do so because they don't want to worry about investing the money. They want to maintain their lifestyle for the term of the annuity. In fact, our past practice of equal installments did not really meet the needs of these winners. Going from an income of say, $50,000 a year to say, $1 million per year, sounds great (and I hear that it is), but ten years later, the winner's income is still $1 million a year and the price of luxury cars and yachts has gone up. In twenty years, the fact that the winner is living on a "fixed income" really starts to sink in. If you took the same cash jackpot amount to a professional financial advisor, they would certainly recommend investing for a graduated annuity so that your "real" income would stay at the same level every year. The Powerball annuity jackpot now does that.
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That is kind of unfair to winners of record jackpots in which a match 5 bonus pool is created though. The MUSL should instead cap its current record cash jackpot and limit those increases to say $12 million per drawing or so.