Beside the exchange rate affecting the maintenance fee, that's how mining works. If you had 1 GH/s and the network hash rate was 100 Gh/s you would have 1% of the network hash rate. Around 3600 BTC are mined each day, so if you controlled 1% of the network you would receive 1% of that 3600 BTC. Now if the network increased to 200 GH/s you would only control 0.5% of the network hash rate and would receive 0.5% of the 3600 BTC.
So, if the network hash rate increases, your payout decreases.
Interestingly, it appears the hash rate is dropping. Difficulty is at 40.3B and while there is still 12 days before the next revision, if it happened today it would drop to 37.1B. Yesterday the estimate was 38B. Not sure what is going on, and it can certainly change, but this is the first time I have seen such a drop. Even in an estimate.
This spreadsheet shows the past difficulty changes.. The network has decreased a few times.