Bitfinex underpaying swap payments

Hey
I recently noticed that bitfinex is underpaying my swap payments. I've sent them a mail 7 days ago, still no response.
Here's my history_USD.csv and my margin_swaps.csv. So as you can see, I started with around $7050. The full balance went to a 30 day swap at 0.0649% daily rate. Only $50 of this swap already closed and got lend out again immediately. You can see my current position here. The interest I make with these swaps gets lend out as well, so my daily earning should go up slightly with time when rate stays the same. Now the math.

$7050 * 0.000649 * 0.85 = $3.889

It's multiplied by 0.85 because they take a 15% fee. So I should get at least $3.9 when money is lend out at this rate, since the interest gets lend out as well, this value went up a little as you can see in history_USD.csv. All went well in the beginning, but then after 10 days or so they start paying me out less and quite irregular values. All the money was still lend out, $7000 of initial balance never even closed. So what's going on here? Are they selectively scamming people with their swap payments?