FD Payment allocations

SparciaM

I have a FD Gold card with two BTs on there. One for £1850 and one for £500, to utilise the 5.9% 5 year deal.

I'm trying to figure out my payment allocations. At the moment I pay the minimum which is 3% and it states that 'payments will be allocated to BTs, Purchases and Cash Advances in that order after any fees, interest and charges have been paid. Balances added to the account first will be paid'.

So they obivously pay the 5.9% interest on the two BTs and then they allocate what is left to the BTs and according to the bottom bit they would allocate all of what is left to the £1850 BT as this was done on opening and then £500 was done a few months later.

My statements show though that they allocate payments in a random order and percentage to whichever BT they like.

So in Oct the interest was (for the BTs), £8.56 and £4.65, but then in Nov it shows as £9.20 and £2.45.

1) How do the interests change? If the balance is actually going down? I would thought the interest would go down as well. (No other payments are being made to the account).
2) Why would the randomly choose to say pay £50 off the high BT and then £10 off the low one month, and then £40 off the low and £20 off the high?

I was thinking of calling to clarify the situation, but just wondering if it is worth the hassle (as I'm guessing it would take a while), or just leave it (as the balance is decreasing).

Any thoughts would be greatly appreciated.