Okay to be honest, I didn't fully understand the google explanation of mutation. Thanks for the clarification. I solved it without now.

I take the list-elements as they are, no casting or anything.

but still, as I add the numbers as a person would do (not a computer) the answer is still wrong (Gryffindor wouldn't win when it's 100 points behind)

I believe in python that your random tests don't always work. I copied some of my output:
Testing for [50, 'yes'] and [300, 'no'] -> after the end of the game it's 200 vs 300
It should work for random inputs too: 'Slytherin wins!' should equal 'Gryffindor wins!'
Testing for [200, 'no'] and [50, 'yes'] -> here it's 200 vs 200
It should work for random inputs too: "It's a draw!" should equal 'Slytherin wins!'
Testing for [300, 'no'] and [50, 'yes'] -> and here 300 vs 200
It should work for random inputs too: 'Gryffindor wins!' should equal 'Slytherin wins!'