[suzumer]

While one commenter had it somewhat right that HDR has to do with how bright/dark an image can be, the main thing HDR images specify is how far ABOVE reference white you can display. With srgb, 100 percent of all channels is 100 percent white (brightness of a perfect lambertian reflector). Rec 2100 together with rec 2408 specify modern hdr encoding, where 203 nits is 100 percebt white, and above that would be anything brighter (light sources, specular reflection, etc). So if a white image encoded in sdr looks dimmer than hdr for non specular detail, that is probably encoding or decoding error.