Because in Python 1 == True (and hash(1) == hash(True)) and you have 1 in your set already.
Imagine this example:
example1 = {0, False, None}
example2 = {1, True}
print(example1)
print(example2)
Will output:
{0, None}
{1}
First set has 0 and None because 0 == False but 0 != None. With second set 1 == True so True isn’t added to the set.