In PWR_DOWN mode, there's 3 things that can be on: WDT, ADC, BOD. The ADC consumes about 90-100 uA. BOD consumes about 17 uA. And WDT consumes about 4 uA. I believe there's something else is consuming rather than these 3 possible factor as if you add those 3 up, it would still let you run more than a day.