I think Delta, have "list" all the usage of this SkyBonus account and calculate the cost of each transaction.
They guy are a travel agent, he must "optimize" the usage of the account.
I use the SkyBonus voucher only in peak period,
Example: In high season, the fare class are available, but the price is more high than normal. I use the voucher in this situation (I use Trans Pacific at 180K).