About realsense precision

I use a single realsense d415 to cooperate with the turntable for reconstruction. The reconstruction process is that people stand on the turntable statically and then start reconstruction. At this time, the camera is located in the upper body, and the turntable is rotated. After the camera collects the upper body for a circle, it slowly moves down to the lower body to continue collection. There is no problem with the reconstructed model, but there is a problem with the accuracy. For example, in reality, the neck circumference is 370mm, but the neck circumference of the obtained model is indeed 420mm, which is very different. I want to know what the problem is. Please tell me what to do.