This is very interesting but I don't understand the raw numbers.
My witness has only been going for a few months so all my votes would be less than 12 months old on 1 July 2021. I currently have 17,418 MV but this simulation would have me with only ~13,000 MV.
Is this saying there is 4,400 MV worth of proxy votes that would expire?
Exactly. One of the voters who vote for your witness has an inactive proxy, which will expire on July 1st unless they update it.