Because D: Momentary is described in an unequivocal way: "The spell lasts but a moment and then disspates.".It is an explicit exception from this, that ""Ritual Creo spells with Momentary duration create things that last as any other thing of that type. The magic is gone in a moment and so cannot be dispelled. This also applies to ritual healing spells."
For a D: Mom spell which is not a Ritual, another such exception is needed to have it create "things that last as any other thing of that type." There is no need to write it all over the book.
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