One piece of dust on the disc can cause the results to be different.
Even with exactly the same disc, there will be slight differences in the vibrations of the spinning disc, tiny power fluctuations in your machine, the physical size of the disc due to temperature fluctuations, etc.
Therefore you will never get identical scans from the same disc. Of course for high quality discs, they will always give scans (on the same scanning drive) that are pretty similar. That seems to be the case right here.