Which photographer can help me explain why I took two photos with different effects and the same light source? What is the reason?

I also like shooting, but only as a hobby. I feel that the effect of the upper and lower films is different, mainly because the exposure is different. Although your position, light source and time are the same, the aperture size and shutter speed can change the exposure. The pure white of your last film is obviously high-profile If you try to increase the exposure parameters of the following photos (1 ~ 2 files), I think you can achieve a pure white back. What you said is not necessarily right, it is purely an opinion.