The batch simularity doesn't surprise me at all. At the beginning of the generation the randomizer is seeded with the current time, It then generates numbers starting from that point using the internal randomizer. Essentially every time you start a new generation batch, regardless of how many you re generating, each batch will get a new seed, but each file in the batch is seeded from the file before it. All random number generators work the same, some apps re-seed each time you need a new number, This one is re-seeding each time you start a new batch.