Bitwise or works at the level of bits, so the question is more like, in:

    >>> bin(0b011 | 0b010)
    '0b11'

Does the leading 1 come from the first argument or the second argument? It isn't really a sensible question for bits, since the 1s are indistinguishable. However the values in the hash table are distinguishable, so it's a valid question.

Also, taking an example from another comment:

    >>> {0} | {False}
    {0}
    >>> {False} | {0}
    {False}

This is another example where the union operator is used, but here it picks the first value, rather than the second value.