I think the reason some airlines have 110v in first/business while 12v in coach simply means they've upgraded the seats only in the upper cabins.

Just FYI, the "110v AC" is also nominal. You can see anywhere from 110-125v. In Japan, the standard is 100v. This isn't a problem with most laptop chargers, which support normally 100-250v. This can be a problem with adapters for other devices (like a CPAP), so it's important to check the rating on the plate.