So... It seems like "Created By" cards have memory of what they should be, even if they do not show like that. For example, here, Diamond Zephrys created a Golden Brightwing who also created a Signature The One-Amalgam Band. This also tells that Diamond is equal or higher in rarity acording to the game to Signature Legendary cards. Might be already known, but It is the first time I experienced this in game and did not know about this before, just knew that signature cards could create other signature cards.