When I enable TextureStreaming, the QualitySetting.masterTextureLimit is abnormal

I have a texture that enable generate mip map and texture streaming, and my MemoryBudget is 512MB, it is absolutlely enough; my MaxLevelReduction is 2; my QualitySetting.masterTextureLimit is 0. But when I set masterTextureLimit from 0 to 1, the texture's mip map level had change to 3. but it should be 1. Next, I change masterTextureLimit from 1 to 0, the texture's mip map level had change to 2, it didn't return to 0, I don't understand why mipmap level is two more than masterTextureLimit, when I set MaxLevelReduction to 3, the mipmap level is three more than masterTextureLimit, But my MemoryBudget is enouth. I want an answer.