[ryanmonroe]

The wording in the original comment was too strong, I've edited it. It's probably not best to consider it a plain "error" since this calculation is actually a typical one provided in finance. It's just that you usually look at other stats too rather than just this one, which gives you an idea about its accuracy wrt realized return e.g. Sharpe, Max Drawdown, Skew, Kurtosis

[hervature]

No, you were correct, arithmetic mean of returns is not standard in finance. If it was, financial crashes would be much worse. They do arithmetic means of "log returns" because 1/n sum(log(r)) = log(product(r)^(1/n)). That is, in log world, the arithmetic mean is the geometric mean.