Passing a string-type number results in a negative number for function outcome

Hi all, first just wanted to mention that I understood the lesson fundamentally, I'm just curious about why javascript accepts certain values in strings - in other words, I don't need help with this problem, I'm just curious if anyone has insight into WHY it results in the way it does below. Thank you!

const numGenerator = function(upper = 10,lower = 1){
  if ( isNaN(lower) || isNaN(upper) ) {
      throw Error("Both arguments must be numbers");
  }

  return Math.floor(Math.random() * (upper - lower + 1)) + lower;

};

console.log(` ${numGenerator(10,50)} is a random number `);
console.log( numGenerator(40,80) );
console.log( numGenerator(100,"300") );

The following console.log statements will then display, in order:

18 is a random number

67

-128300

I understand passing a string into a function that's expecting numbers will usually result in an error message, but that's if you type out the number itself (example: when I typed out "three hundred" then an error message displayed, but when I type "300" with the quotes it just results in a negative number every time.

I understand the code overall, I was just wondering why typing console.log(numGenerator(100,"three hundred") results in an error while typing console.log(numGenerator(100,"300") results in a negative number.