(NEXSTAR) — Ahead of Saturday’s Powerball drawing — which would produce two jackpot-winning tickets — the game’s grand prize was an advertised $1.8 billion. But, after the dust settled and those winning tickets were confirmed, the prize was said to be about $13 million less than that.

So what changed?

When it comes to Powerball (and Mega Millions) jackpots, a winner gets to decide between receiving their prize as 30 annuitized payments or a one-time cash payout. The numbers you see advertised with the game represent those two options. The larger of the two, $1.787 billion with this jackpot, represents the annuitized option, while the smaller, $820.6 million in this case, is the cash value.

The cash value listed for the jackpot is, generally, how much is needed to fund the estimated jackpot annuity prize, Powerball officials explain. Ticket sales estimates are used to project the cash value of jackpots, as are interest rates, specifically U.S. Treasury bonds, which are purchased to fund the annuity prize payments.

While Powerball officials haven’t said what caused the jackpot to come in under the $1.8 billion advertised value, and a spokesperson did not immediately respond to Nexstar’s request for comment, it’s not uncommon for that number to change between drawings.

Sometimes, it’s due to ticket sales. The majority of sales happen on the day of the drawing. If real sales come in under predictions, though – external factors, like weather, can influence this, for example – or if there are changes to the U.S. Treasury rate, the pot’s value can change.

But, when the jackpot is over $1 billion, is a difference of $13 million really that much?

Unfortunately for the winners of the current jackpot, it might be. Because two tickets — one each in Missouri and Texas — matched the numbers drawn Saturday, the ticketholders will be splitting the prize. According to Powerball officials, the winners (who may never be publicly identified) will receive either $893.5 million with the annuitized option or a lump sum payment of $410.3 million.