I think there is an error concerning the information about supply voltages. At least it's not consistent: microchip has separate websites for ATmega328 and ATmega328p. On the site for the ATmega328p it says: "The device operates between 1.8-5.5 volts". But the datasheet they link on the exact same site says:
"Operating voltage: 2.7V to 5.5V for ATmega328p".
This is not just confusing, but contradictory and so I think the only one who can say for sure is the manufacturer.
Actually, the specified supply voltages are determined by the manufacturer by testing at various levels, but the device might operate even below or above this range (of course, a developer has to comply with the specs). The 2.7V rating could be an old spec and in the meantime they might have found out that the device indeed runs fine even at 1.8V. Maybe, they have just missed to apply this change everywhere this rating is mentioned, but that's just a guess.