Explained under 'If you pay by Direct Debit' here:
https://www.ovoenergy.com/energy-bills-support-scheme
The reason it's done that way is because, if your energy supplier has done their sums right, the amount that you're paying by direct debit should, in itself, already be sufficient to cover the costs of the energy that you use. There's thus no need for them to hang onto the £66/£67 payments, so they're then refunded back to your bank account.