This article also answers the question: why did the maintenance and electricity fee increase together with the Bitcoin exchange rate?
To answer this question, it is required to understand how the fees are calculated and what causes the differences. Let's look at an example: July 14 and 15, 2015.
Datetime BTC Log Price BTC History 15.7.2015 0:00 283.2 285.28 14.7.2015 0:00 285.8 283.2 13.7.2015 0:00 307.148 285.8 12.7.2015 0:00 287.998 307.15
The first column displays the date and time of a log entry by GMT +00:00.
The second column contains the USD/BTC exchange rate logged in our system at the time from the first column. The data is taken from BTC-e via their API. It is possible to check the data we receive by following this link: https://btc-e.com/api/3/ticker/btc_usd (It is the exact same link our system sends the query to).
In the third column are the historical numbers from http://www.investing.com/currencies/btc-usd-historical-data (in USD/BTC).
The difference in values is noticeable. This is caused by the fact that historical data takes the price within the whole day while our system, when making the calculations, uses the price of Bitcoin at the specific moment of the payout and fee deduction (they happen at the same time - at midnight by GMT +00:00 plus 1-2 minutes, depending on the queue position of processing a specific account).
As an example, if we take 1 TH/s of SHA-256 that causes a deduction of 1 USD, then the values will be the following:
Datetime BTC Log Price Maintenance & Electricity Fees 15.7.2015 0:00 283.2 0.003531073 14.7.2015 0:00 285.8 0.00349895 13.7.2015 0:00 307.148 0.003255759 12.7.2015 0:00 287.998 0.003472246
This time the third column shows the cost of 1 USD by the USD/BTC exchange rate at the moment of a payout. It is visible that the fee is smaller on July 14 while the historical data may show the opposite.