An add-on rate quotes interest as a percentage of the purchase price of a money-market instrument, not as a percentage of face value. That makes it easy to compare instruments that already use interest-over-price quoting. T-bills, by contrast, are often quoted on a bank-discount basis (interest relative to face), so you may convert them to an add-on equivalent before ranking yields. The trap is ranking quotes that use different bases as if they were already comparable.