For example, we know some PA voters received two ballots (issue was corrected): https://whyy.org/articles/glitch-sends-some-pa-voters-duplic...
Others received incorrect ballots and had to get mailed a second, correct one (issue was corrected): https://www.post-gazette.com/news/politics-local/2020/10/14/...
Whether this is responsible for the discrepancy, I have no idea. Of course, it's also possible that the mail date is wrong, or that we're misunderstanding what it means. Someone else floated possibility of issues with military ballots. My hope is that somebody calls and asks, because it could be a simple explanation. I don't see any reason to assume the worst. Props to the blog author for great work.
Of course she shredded and discarded the 2nd one.