I know that my logic is missing something crucial as I always assumed that the longer you leave your money for the closer it approaches to the long term average return.
I was just reading this question and one of the answers states that:
"Buying the S&P 500 Index is a wise decision. It is a benchmark and you are basically buying the whole market. Not sure what you mean by 'considerable returns', but the average return for 3, 5, 10, 15 years is 9%, 8%, 13% and 7.6% respectively. This data is from Morningstar. To expect much more than about an 8% return over the long haul is probably not realistic. Hope this is helpful."
I guess this is some sort of a statistical artifact stemming from give set of datapoints.
If you can get 9% a year for 3 years, but 7.6% a year for 15 years couldn't you just do the 3 year hold 5 times?
I tried finding original Morningstar article for extra clarification, but was not able to find it.