CPython is looked to as the canonical Python. IronPython and PyPy are all modeled after it and aim to behave as closely to it as possible, while CPython is considered the gold standard. Comparing CPython3 and CPython2 is orthogonal to that; one is not trying to implement or emulate the other. You have similar situations with Mono C# imitating Microsoft C# and IronRuby imitating Ruby. If there is a difference, it is considered a deviation from Ruby which is the reference implementation.