12V is only a recommendation that has been used for quite some time. There's no real absolute reason 12V was picked, other than to help limit what a new user would choose. So really, all that one cares about is not putting a higher voltage than would damage the regulator itself.
The Arduino ADK board uses a LM2736 regulator, which has an input range up 18V:
Looks like whoever designed the box decided to put the absolute maximum of the regulator. So the real issue is that the ADK's description page says max is 20V, which was probably a result of copy/paste error. The difference between saying the max is 12V or 18V is irrelevant. One is a real limit, the other is a suggested limit.
This is why I said there is nothing wrong with the packaging. What is printed is (technically) correct.
As was stated, if you use 18V you'll get less current through the regulator before it shuts down, compared to 12V or 9V or 8.5V or 7.65V, etc.
It should be noted that the ADK and "standard" MEGA 2560 board differ on the 5V regulator. The "standard" uses a NCP1110 from OnSemi which does have a max input range of 20V.