Why do rows still exist for a ledger and period in the GL_BALANCES table after running the purge?

I am following the standard purge process for a specific ledger and period and permanently closed the period first, via a SUPER_USER responsibility.

I did a count on the data in the GL_BALANCES table for the specific ledger and period BEFORE the purge, and it was around 60000 rows. I ran the purge balances process and it completed successfully, and then I checked the row count again and found that it had deleted around 30000 and left 30000 still in the table.

By comparison, when running the journal purge for the same ledger and period, there were 80 rows in the GL_JE_HEADERS table before the purge, and 0 rows after the purge.