Well her ancestor did lie to the mountain god and said he was the one who played a trick on that girl. Even the mountain god didn't know he was lying until after he placed the curse on him. He probably lied to his family about what actually happened well. Plus over the course of 300 years it makes sense that the origin story of the curse would change. From one generation to another the story probably lost details and tweaked until what she heard when she was young.
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.well, the mountain god, the tiger guy who is the lead males friend erased his memories in turn for the dog curse to be lifted from the girl and her family, but she had to chose between her family and her lover so she in the end chose her family so that's why and how he lost his memories. You just have to reread a little bit if you got lost. (●'◡'●)ノ

so what I want to know is like how did the story change so much? Wasn't it said that their ancestor int he beginning was a troublemaker and killed a dog and the god got angry but that's not the truth, so how did it get so weird?