This is due to the fact that when you iterate a Dictionary (e.g., items) or Set, the iteration order is based on the order of the elements in memory — which depends on their hash value. These hash values are based on a seed which is randomized on every run of your program, leading to a different iteration order every time you run the above code.
If you print(values) inside of your outer reduce, you'll be able to see that the order they're passed in changes — and because result compounds here, this affects the final result.