Cars don't look exactly the same and are not in the exact same places. Lighting changes. People look very different and walk different paths. It's going through PRNG so even very small pixel differences (which you can find even on an empty street thanks to changing weather, litter, wind and so on) are good enough if they are unpredictable. In other words, when you compare pictures of the same lava lamp with pictures of some random cars, I think you will be able to find more patterns in the lava lamp movement. You would also likely be able to predict it better (well that's the same thing I guess).