[shdon]

Decoding JPEG doesn't leave much room for interpretation and the images should essentially always be decoded the same. For encoding, it's a different story, as there are steps to downsample the image data (chrominance data is often, but not necessarily, sampled at a lower rate than luminance data), there can be a different cutoff point for which of the DCT coefficients to discard (usually related to the "compression" or "quality" setting for the compressor). All JPEG decoders should reproduce the same image from a given JFIF file, but I'd be surprised if different encoders produced the exact same JFIF file from a given source image.