Hi panic ,
From the link "With specific hardware configurations, Windows refuses to let you change the Critical Battery Level below a specific point. If you’ve got a big battery, this prevents you from using every last bit of juice. Thankfully, there are two easy workarounds.
We’ve already shown you How To Tweak the Low Battery Action on Your Windows 7 Laptop, but Windows can be stubborn. On my particular laptop, it won’t let me set the critical battery level to anything below 5%; it changes back to 5% as soon as I click something else. On my netbook, that’s close to 20 minutes I’m missing out on, and it takes less than 30 seconds to hibernate so I know I would be fine with 1%. Depending on your specific hardware this number may be different, or you may be lucky and not have this problem at all; it appears that many Macbooks running Windows 7 don’t have this issue. If you’re like me, however, there are two solutions you can choose from: one that’s easy and another that’s slightly more involved."
It might be the case that your hardware doesn't allow it, or it could be something in either Control Panel --- Power Options --- Change when the computer sleeps/hibernates.
Or it may be a function elsewhere within the Advanced Power Options menu tree that is 'resisting'.