Why are these two If-statements not the same?

I just started learning Javascript. Currently I'm taking an online course and I just ran into my first problem I do not understand.

I want to check how many guesses I still have in a Hangman game:

const Hangman = function (word, remainingGuesses) {
this.word = word.toLowerCase().split('')
this.remainingGuesses = remainingGuesses
this.guessedLetters = []
}

Hangman.prototype.getPuzzle = function () {
let puzzle = ''

this.word.forEach((letter) => {
    if (this.guessedLetters.includes(letter) || letter === ' ') {
        puzzle += letter
    } else {
        puzzle += '*'
    }
})

return puzzle
}

the correct If statement in the video is this:

Hangman.prototype.makeGuess = function (guess) {

const isUnique = !this.guessedLetters.includes(guess)
const isBadGuess = !this.word.includes(guess)

if (isUnique) {
    this.guessedLetters.push(guess)
}

if (isUnique && isBadGuess) {
    this.remainingGuesses--
}
}

But this is below here how I wrote the if statement:

Hangman.prototype.makeGuess = function (guess) {


if (!this.guessedLetters.includes(guess)) {
    this.guessedLetters.push(guess)
}

if (!this.guessedLetters.includes(guess) && !this.word.includes(guess)) {
    this.remainingGuesses--
}
}

The remaining guesses are not calculating correctly, if i do the if statements with the second way. Can you please tell me what is the difference?