I would venture to suggest it's because they don't know the risk involved beyond 2 x SD. If they ran a 3 (or more) SD adverse move together with a 30% IV shift I think they'd no longer be describing their 10% monthly returns as "safe".Quote from atticus:
Why is it that the "safe 10% monthly returns in any conditions" types never seem to mention their risk beyond two deviations?
A 10% per month return is certainly achievable, but it's far from safe.