There is one critical flaw in that calculator.

The difficulty is increased at 30 day intervals. Even if set at 40% that is not reflective of reality.

If you decrease the interval to 14 days and 20% for that period, you'll see the end number change quite drastically.

I know, because I created an excel sheet to do this. I included the ability to change the period. The frequency of the difficulty change makes a dramatic difference in the overall sum of btc collected over the course of a year.

The calculator is (really good) but it doesn't have a realistic frequency for difficulty changes. So the numbers it gives are drastically different than what it should be.