Why would different characters impact the order of the output for this getPositions method?

Question

class Solution(object):
    def getPositions(self, s):
        mapping = {}
        for i in range(len(s)):
            if s[i] not in mapping:
                mapping[s[i]] = [i]
            elif s[i] in mapping:
                mapping[s[i]].append(i)
        return mapping.values()

I would expect

getPositions("paper") == getPositions("title")

to be true but the output actually ends up being:

[[1], [0, 2], [4], [3]]   ==   [[1], [4], [0, 2], [3]] 

Shouldn't the values of the character in the strings not matter in this method and what is causing the change in expected output?