It does sound suspicious, but it works out. Let's say you have $100k to invest, have a 30% tax rate, and your investment will give you a 2x return by the time you're ready to withdraw.

Pre-tax investment: Invest $100k and it becomes $200k. You pay $60k in taxes and end up with $140k. Post-tax investment: You pay $30k in taxes and invest $70k. That $70k becomes $140k.