There are some small amounts of HBD transfers from the @hbdstabilizer to the @hive.fund. These are most likely the beneficiaries rewards

The beneficiary rewards are treated the same way as DHF funding, so HBD is traded for HIVE as long as HBD is overvalued.

The reason some HBD got sent back is there were periods when HBD was not overvalued. So under those conditions, HBD received from DHF and beneficiaries is sent back right away, and HIVE that ends up in the account (mostly from power downs of HP beneficiary rewards) is used to buy HBD, which is then also sent back.

Also, i think the graph after that paragraph is missing.