Since when does difficulty stay still? I actually anticipate a massive spike once Asicminer puts their 15 PH on the network.
You're using the wrong calculators dude, no way you'll mine 15.622 BTC under any conditions.
Um... simple math:
Difficulty * 2**32 / hashrate / 86400 = number of days to find a block.
25 / number of days to find a block = expected earnings per day.
Expected earnings per day * 365 = total BTC earned over a year.
11756551917 * 2**32 / 1000000000000 / 86400 = 584.42136571
25 / 584.42136571 = .04277735
.04277735 * 365 = 15.6137344
So, yes, assuming static difficulty, you will indeed earn that amount. If you anticipate a 10% increase in difficulty, then you repeat that formula 26 times (assuming a 2 week difficulty change - this one I'll give you is variant because of reliance upon 2016 blocks, rather than actual time).
You always calculate with BTC invest in, never fiat.
If you invested 4 BTC, you ROI by making back 4 BTC plus, not what 4 BTC's fiat amount was worth when you invested.
That's like saying I paid 4 gold bars for this house, but the house tripled in value, so the 1.33 gold bars I made back is A-OK. Uh, no it's not, you just lost nearly 3 gold bars.
Your example is wrong. You're converting gold bars into a house. The example would be better had you said, "I gave a guy 4 gold bars hoping that after a year he would return me more than 4 bars."