I receive different results with my code. For the same code, most of the time assertion is completed but result is saying error in "One" test. Any clue? Thanks

From: http://www.checkio.org/mission/most-wanted-letter/solve/

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Mozilla/5.0 (Windows NT 6.2; WOW64; rv:32.0) Gecko/20100101 Firefox/32.0

code follows

def checkio(text):
    import re
    from collections import Counter
    cnt=dict(Counter(re.findall("[a-z]",text.lower())))
    lst=cnt.items()
    srt=sorted(lst,key=lambda k: k[0])
    print(srt)
    srt=sorted(lst,key=lambda k: k[1], reverse=True)
    print(srt)
    print(srt[0])
    print(srt[0][0])
    return srt[0][0]
if __name__ == '__main__':
    #These "asserts" using only for self-checking and not necessary for auto-testing
    assert checkio("Hello World!") == "l", "Hello test"
    assert checkio("How do you do?") == "o", "O is most wanted"
    assert checkio("One") == "e", "All letter only once."
    assert checkio("Oops!") == "o", "Don't forget about lower case."
    assert checkio("AAaooo!!!!") == "a", "Only letters."
    assert checkio("abe") == "a", "The First."
    print("Start the long test")
    assert checkio("a" * 9000 + "b" * 1000) == "a", "Long."
    print("The local tests are done.")