Hi
The power consumption is likely due to the voltage regulator that is dropping 12 volt down to 3.3 volt. It is a linear voltage regulator I believe and they are not very efficient.
You would need to go for a self build design I should think to remove the linear voltage regulator and replace with a very efficient switch mode design.
The low power consumption figures quoted for these processors when operating or in sleep mode are from the 3.3 volt side, they ignore how that voltage is sourced and what power consumption is used/wasted dropping the voltage down.
A self build design omitting all the voltage regulation stuff and running from a couple of AA batteries using sleep modes would then see the consumption quoted in the datasheets.
Regards
Phil