Writing Lucky Sevens, what is wrong?

I'm writing Lucky Sevens and I'm getting mostly expected results based on how dice games work IRL. But every once in a while I get a crazy result. Is this the small probability of random number generators or do I have something wrong?

Here's the Js:

    function rollDice() {
      return Math.floor(Math.random() * 6) + 1;
    }

    function playGame() {
      var startingBet = document.forms["luckySevens"]["startingBet"].value;
      var gameMoney = startingBet;
      var die1;
      var die2;
      var diceSum;
      var rollsNumber = 0;
      var moneyMax = 0;
      var rollsAtMax = 0;

      if (startingBet <= 0) {
      alert("Starting bet must be greater than zero!");
      return false;
      } //end of alert if loop

      while (gameMoney > 0) {
       die1 = rollDice();
       die2 = rollDice();
       diceSum = die1 + die2;
       rollsNumber++;
       if (diceSum != 7) {
        gameMoney = gameMoney - 1;
       } else {
        if (diceSum == 7) {
         gameMoney = gameMoney + 4;
         if (gameMoney > moneyMax) {
           moneyMax = gameMoney;
          rollsAtMax = rollsNumber;
          }
         }
        }
       }
       document.getElementById("results").style.display = "block";
       document.getElementById("playButton").innerText = "Play Again!";
       document.getElementById("betStart").innerText = startingBet;
       document.getElementById("rollsNumber").innerText = rollsNumber;
       document.getElementById("moneyMax").innerText = moneyMax;
       document.getElementById("rollsAtMax").innerText = rollsAtMax;
       return false;
       }//end of playGame()