Years ago, a marketing type looked at a solar panel's data sheet, picked the absolute highest voltage the panel would ever put out, which happens with no load ("open circuit" voltage). He then picked the absolute highest current anywhere on the data sheet, which happens with a dead short (basically an infinite load; the exact opposite condition from the maximum voltage condition). He then multiplied those two numbers together and decided that it was an "80 watt" panel. Never mind that the absolute maximum power you'll ever get in the real world, somewhere between zero and infinite loads, is maybe 60 watts...his fictitious number looked better so he used it in his advertising materials.
Other manufacturers were faced with a dilemma. They could either do the right thing, rate their panels realistically, or cheat like the first guy did. But rating the panels realistically isn't an option -- who'd buy a 60 watt panel when someone else sells an "80 watt" panel for the same price? So all the other manufacturers had to fall in line and use the same shady rating technique whether they wanted to or not.
Battery manufacturers do the same thing, again whether they want to or not. If you multiply the amp-hour capacity of the battery on my 2014 Zero SR (100 amp-hours) times the nominal voltage of the battery pack (somewhere around 100 volts), you find the battery pack is nominally about a 10.0kWh battery, which Zero informally says is pretty accurate. But if you use the absolute highest possible battery voltage you can charge the battery to without damaging it (114 volts), you get the "rated" number that Zero advertises, 11.4kWh. It's cheesy at best, fraudulent at worst, but they HAVE to do it that way because everybody else does. Otherwise, the customer would be comparing apples to oranges, which wouldn't fairly compare Zero's specs to other manufacturers'.