It's byte for byte identical once you expand to an array of pixels. The storage format uses more advanced (lossless) compression, but the pixels that come out are identical.
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.