The reason of not withdrawing more than available is the way the formula calculates the expected drawdown. The drawdown grows with the square root of number of trades, while the profit grows linear with the number of trades. Thus we're using the profit as a proxy of number of trades in the formula, just for simplicity's sake. But this assumption is violated when you increase the margin for reinvesting, as the profit then increases with a higher exponent. It's also violated for the same reason when you decrease the margin by withdrawing too much. In the former case the formula errs on the side of caution, in the latter case it errs on the side of risk. That's why you can withdraw less, but should not withdraw more than what's available.