I always thought that the 'UPTO 2 weeks' meant something like, "If you read for an hour before bed, it'll last 2 weeks". In my experiences UPTO estimates always use numbers based on liberal usage like I mentioned, the not-so average grandma reader
The 8000 page turns is probably also a naive calculation, where they calculated how much power it takes to do a single page turn, and divided the average battery capacity.
This kind of simple calculation would ignore the fact that nobody is going to do 8000 consecutive page turns like that. Which means it doesn't account for additional drains on the battery:
- Power down, Power on (at least a couple screen updates, plenty of internal memory reads, processing, etc)
- ANY other operation besides page flips will usually include a page refresh + processing (display tab to check battery, change font, browse library, GOTO next chapter, etc)
- even powered down, the kobo has to use minimal battery power to keep the clock/date system going
Technically speaking, it can do 8000 page turns, if you ignore all that crap, so they can put it on the box without getting sued. Same goes for the 2 weeks... it can last two weeks, sure, but it won't be anywhere near 8000 pages since you also have all the battery drains I mentioned coming into the equation.
Notice specifically what they say, note the 'or' & 'approximately':
"This charge will last approximately 8000 page turns or two weeks of average reading."
Marketing people get payed a lot of money to brain-f**k you, and they do it well. Sometimes they get sued for false advertising, sure, but I think Kobo is safe by their careful wording.