Keep in mind that they are often wired improperly, and often not programmed correctly, and not rezeroed properly, but the human watching it might still see 92% and get that seratonin filled headrush and a burst of confidence, when the batteries are 74% and aching for a full recharge.
But when they are wired properly, recently rezeroed, well I love seeing a XX AH from full, and when charging my battery watch it reset to ) from full right when the battery is actually full, determined by a hydrometer or determined by how much current the AGM is accepting at absorption voltage.
Heres some good reading on battery monitors:
http://www.pbase.com/mainecruising/battery_monitorhttp://www.pbase.com/mainecruising/programming_a_battery_monitorhttp://www.pbase.com/mainecruising/smart_gauge