1 million ballots at 10,000 per batch means 100 batches. It seems they will use 400 ballots from each batch as a sample, and if anything is wrong for one of those 400 sample ballots, then the entire batch of 10,000 is sent back. Some nefarious actor could find 100 messed up ballots and include one in each sample of 400, and viola! 100 ballots invalidates 1 million votes. That's quite a system they got there.