When you transcode back to JPEG, the JPEG is byte-for-byte identical to the original JPEG. But if you decode to pixels you do not always get identical results a

When you transcode back to JPEG, the JPEG is byte-for-byte identical to the original JPEG. But if you decode to pixels you do not always get identical results as the JPEG because the JPEG standard does not specify bit-exact decoding and different decoders may differ slightly.