This seems to be the issue. Running it 10 times gives me quite consistent performance near the top.

    0 - Passes: 5876, count: 78498, Valid: true
    1 - Passes: 5870, count: 78498, Valid: true
    2 - Passes: 5944, count: 78498, Valid: true
    3 - Passes: 5903, count: 78498, Valid: true
    4 - Passes: 6051, count: 78498, Valid: true
    5 - Passes: 5874, count: 78498, Valid: true
    6 - Passes: 5902, count: 78498, Valid: true
    7 - Passes: 6041, count: 78498, Valid: true
    8 - Passes: 5906, count: 78498, Valid: true
    9 - Passes: 5889, count: 78498, Valid: true

Will run it 100 times now to lower the risk that this is just a coincidence.