Latitude Payments, can someone explain?

We signed up to Latitude for our big 60 Month Harvey Norman purchases. Since then, we've added a few one off large purchases that you can pay over 6 months.

We recalculate the amount we need to pay each month depending on the terms of each plan.

We've noticed we are now being charged interest on plans that should have been paid off ages ago by Latitude's own explanation:

We automatically allocate payments in your best interest. Payments will go towards the plans incurring/due to incur the highest interest, then to the lowest.

I note at the end of this same page, they say:

For deferred/Buy Now Pay Later terms, these do not require any monthly payments, so no payments will be allocated to this for the duration of the term, unless requested by you.

I don't know what's considered a plan and what is considered a deferred payment. I also think this is made confusing on purpose so they can make their additional revenue through interest.

Can anyone advise? Additionally, I assume this means we need to now manually manage each purchase ourselves and not rely on their system to do it for us.