It's because of the usage of the createOnly action. createOnly will only work when the key is unambiguous. It will work fine if you use createDuplicate, or if you use a different unique key.
Maybe the docs should indicate that ambiguity isn't deterministic (or at least is too complex to document). If you look at Damodhar's 1st and 2nd examples, why is Lead4 accepted in 1 but not in 2?