The most acceptable explanation is the Fibonacci sequence 0, 1, 1, 2, 3, 5, 8, 13, 21, and the number is not 21, but 20999999.9769 BTC.
When I see that number, I feel that I should have add more to make it into a whole number instead of having that .9769 BTC which I think Satoshi also did that and that's why the maximum supply is 21 million BTC. Well, there's nothing to support my claim but it's what I think and my opinion why it is 21 million instead of 20999999.9769 BTC. I also believe that it is not based on how many gold bars that are mined already.