Why does this not pass but gives different ordered, but correct results?

I've read this threat, which is explaining why it didn't pass but I still don't understand why.

I used the following code

def word_count(arg):
    words = arg.lower().split(" ")
    words_dict = {}

    for word in words:
        if word in words_dict:
            words_dict[word] += 1
        else:
            words_dict[word] = 1

    return words_dict

The mentioned thread says that it does not split properly if there is a trailing whitespace, which, in theory makes sense. It seems like an edgecase, though.

Theoretically it should of passed but it did not give me an error that stated that there was something wrong with the method, but referenced that the output was not what was expected and suggested I use .lower()

Also, using:

words = arg.lower().split()

instead of :

words = arg.lower().split(" ")

returns the same dictionary, but with a different order.

Why doesn't .split() split every character instead of at spaces when you are not providing an argument?

Also, shouldn't the test have passed? Maybe shouldn't the error given more reflect the issue? possibly something about length of the obj being too long/too short, empty key or ect.?