Its obvious, manufacturers have over estimated the effectiveness of the main battery and have either just saved money on the 12V battery or under estimated the standby drain of all the electronic systems when switched off for circumstance at the end of the customer useage curve for low milage.
It would be a simple matter to monitor the 12V voltage when off and use the larger li-ion battery for a top up, but that would entail even more power useage when off, even if a intermittent timed measurement, say every hour. That power useage might affect the range figure, and that marketing number is now number one for advertising electric, and may just affect the mpg of hybrids too.