Why "if letter in dict" working wrong in Python?

I am trying to write a function, that returns a string letters that are not in lettersGuessed

words = list('abcdefghijklmnopqrstuvwxyz')

def getAvailableLetters(lettersGuessed):
    [words.remove(letter) for letter in words if letter in lettersGuessed]
    return ''.join([str(elem) for elem in words]) 

then i call my function

getAvailableLetters(['e', 's', 'i', 'k', 'p', 'r'])

But the result is different from what I expected. My output:

'abcdfghjlmnoqstuvwxyz' 

# the letter 's' in the output, but it shouldn't be there.

Correct output:

'abcdfghjlmnoqtuvwxyz' 

what am I doing wrong ?