There are several jpeg decoders like ijg, libjpeg-turbo, mini jpeg decoder, … Assuming that the encoded jpeg file internal color format is YCbCr, to produce the final RGB image, the decoders have to do color space conversion. All these decoders are using the equations given in https://www.w3.org/Graphics/JPEG/jfif3.pdf even if the ICC block of jpeg image is indicating otherwise. What is the reason behind this?