It may seem a bit counterintuitive, but the pair of LEDs at the top left (second row) are "input peak" and are not user adjustable. They illuminate at -3dBFS. When you set the "Meter: Peak Threshold" to 0dB, you're actually forcing the far right set of LEDs on the segmented meter to be 0dB. That far right set of LEDs on the 722/744t can be set at something less than 0dB (0-20 I believe) though logically I wouldn't imagine most of us would use anything but 0, -1 or -2.
- Jason