Digital camera takes pictures, the background is black, black belt and dark bag. Why is the background not pure black?

black background, the subject with bright color. If spot metering is used, the exposure parameters are set according to the brightness of the subject. If the exposure in the dark is insufficient, the black background will be "pure black". If you shoot a dark subject, if you still use spot metering, set the exposure parameters according to the brightness of the subject, the shutter speed will be much slower, and the dark place will be fully exposed, so the black background will not be so "pure black".

if average metering is used, the black background will not be so "pure black" even if the subject is photographed with bright colors.