As far as I can tell, the calculator seems to be working fine.
I think you aren't realistic. You are expecting your mining rig to be a money making machine for ever. The problem is everyone realizes the profitability of mining. As you can see in the calculator the difficulty is expected to rise with 110% every month. So after a year the mining difficulty would have risen with roughly 1500%, which would of course impact the profitability of your mining rig.
Some notes on the calculation:
- you are calculating with 0 $/kWh.
- you expect the price of bitcoin to remain at 1100$ for the rest of the year? Seems unlikely. Many miners expect to generate profit due to the future price of bitcoin.
- you should take in mind the mining rig still has value after the mining isn't profitable any more for you. Selling it will increase your profit even more.
Risks are of course potential pricedrops of bitcoin and an even steeper increase of the mining difficulty. No risk = no profit.
Even besides these points, you are generating 3800 USD worth of bitcoin, with an 3000 USD investment: a ROE of 25% + residual value of your rig. This profit is in BTC (assuming you don't immediately cash-out), so on top of this profit you would also benefit of the increasing value of Bitcoin.
http://mining.thegenesisblock.com/a/65573407e1