Supplier credit balance is not the same when I go to apply to outstanding bill

I received 'government energy bill relief' credit on an electricity bill, which then left a credit for several more future bills. The first two bills when I applied the credit, worked correctly.

Now I have a third bill which should be covered by the remaining credit and still leave a credit balance of $1.01 to be applied to the next one. However, it does not.

The credit available is 97 cents short to apply to the current bill.

When I look at the history of the credit entry, that shows the correct credit amount still available. The bill relief does not include GST, but the bills do.

I'm guessing this has something to do with the problem, but I don't know how to resolve the mis-match.

Thanks Danni