[Bug]: "averageTotal" for "Keep" modifiers calculates incorrectly

[JPStrydom]

Description

Firstly, thanks for the awesome tool! Really easy to use and super useful.

I'm not sure about he maths happening behind the scenes for the averageTotal calculations, but I've noticed that the values it returns when the Keep modifier is used looks incorrect.

I ran the following to test the average:

const { DiceRoll } = require('@dice-roller/rpg-dice-roller');

const getAverage = numbers => numbers.reduce((total, number) => total + number) / numbers.length;

const getSuccessRate = (numbers, target) =>
  getAverage(numbers.map(number => number >= target)) * 100;

const executeRolls = (rollTypes, { target = 10, numberOfRolls = 1000 } = {}) => {
  const rollResults = rollTypes.map(rollType => ({ rollType, rolls: [] }));

  for (let i = 0; i < numberOfRolls; i++) {
    rollResults.forEach(rollResult =>
      rollResult.rolls.push(new DiceRoll(rollResult.rollType).total)
    );
  }

  return rollResults.map(({ rollType, rolls }) => ({
    rollType,
    average: getAverage(rolls),
    rpgDiceRollerAverage: new DiceRoll(rollType).averageTotal,
    successRate: getSuccessRate(rolls, target)
  }));
};

console.log(
  executeRolls(['1d20', '2d20', '1d20 + 1d4', '2d20 + 2d4', '2d20k1', '3d20k1'], {
    numberOfRolls: 100000,
    target: 15
  })
);

Which outputs the following:

[
  {
    rollType: '1d20',
    average: 10.49794,
    rpgDiceRollerAverage: 10.5,
    successRate: 30.076000000000004
  },
  {
    rollType: '2d20',
    average: 21.00188,
    rpgDiceRollerAverage: 21,
    successRate: 77.286
  },
  {
    rollType: '1d20 + 1d4',
    average: 13.04473,
    rpgDiceRollerAverage: 13,
    successRate: 42.844
  },
  {
    rollType: '2d20 + 2d4',
    average: 26.01443,
    rpgDiceRollerAverage: 26,
    successRate: 90.688
  },
  {
    rollType: '2d20k1',
    average: 13.81472,
    rpgDiceRollerAverage: 10.5,
    successRate: 50.775000000000006
  },
  {
    rollType: '3d20k1',
    average: 15.47827,
    rpgDiceRollerAverage: 10.5,
    successRate: 65.549
  }
]

As you can see, all the average calculations that the rpg-dice-roller made were correct, except for those using a Keep modifier.

Here's a summary of the Keep modifier's expected and actual average values:

Roll Type	RGB Dice Roller Average	True Average
'2d20k1'	10.50	~13.81472
'3d20k1'	10.50	~15.47827

Dice Roller version

5.5.0

Bundle type

UMD

Node version

20.10.0

Browser name and version

No response