I would assume it has to do with the totla return of a larger pot of money vs. the increase tax obligation of long term capital gains tax rate.
The answer to this question is simple. Assuming that the college investment you are making is the same in amount and timing of investment, in a 529 account versus a personal account, the benefit of the 529 accounts "Tax free" status on all 529 account earnings is the key advantage. Thus, you will accumulate true long-term value over the future years through the annual tax savings feature so you will have more funds available for college. thus, your personal account returns have to exceed the and 529 account returns by the incremental tax rate you will be paying on the earned income. thus, you will have to take a greater market risk when investing. Which is obviously not the best idea with future college needs.
Another thing to factor into your decision...Many states allow for deductions against state income tax for contributions to their (state sponsored) 529 plan. Depending on your state and its income tax rate, this could add to the value of a 529 plan versus a taxable account.
The simplest way to look at it is – Taxable (capital gain & dividends) as you go along, or tax free as long as it is used for education.
Therefore, to calculate the equivalent total return of a taxable account use the 529 return and add to it your tax bracket.
Every 529 plan is different. Work directly with your financial advsior