For BitCoin or any other 'single-coin' mining operation, this is very true.
Difficulty rises on a single popular coin lead to the results your calculator shows.
Scrypt.cc?ref=baagt is currently analyzing the profitability of over 40 profitable scrypt coins every 60 seconds and mines the most profitable 3-5 coins of the moment.
The varying difficulties, current exchange rate and other variables are all considered before mining a coin. Pretty keen piece of software actually. Would love to take an up close and personal look at it.

Because of the nature of the market and so many coins, the profitability for the shares you purchase have a much better payout for a much longer time than if they were tied to any one specific coin/network.
Medium and possibly long term, awesome investment.

Keep it Clean!
