Sounds like the ebook reader I got for Christmas last year. It claimed to have enough battery life to read War and Peace five times, but I was finding it only had enough to read one short novel once. Turned out that when I thought I was turning it off between uses, I was actually just suspending it and it was still using lots of power. The actual off function, the one you have to use in order to make the marketing claim true, is buried several levels deep in an "Advanced Settings" menu.

That seems particularly silly because there isn't any obvious advantage to the vendor in making you use the battery up faster! If they were the only supplier of replacement (non-rechargeable) batteries, you could imagine how they might find it advantageous to suck you in with promises of long battery life and then squeeze you for every drop of juice they could make you consume. <f/x: grinding of tortured metaphors> But unless they're in league with the electricity company, that doesn't seem to work as an explanation.

I strongly suspect that the decision to make it sleep rather than switch off was based on how long it takes to come back from sleep (and what state it retains), and was made by marketing.