There are thousands of member banks so that $1.6 billion gets spread out based on how much Paid In Capital each bank contributed. And if that $1.6 Billion is accurate and represents a 6% interest payment, then the member banks put up $1.6/.06 = $27 billion in capital, for which they are receiving 6% annually or $1.6 billion.
I didn't check their annual reports to confirm your number.