The rest of it is used for system and specially GPU. It's a know fact from day one of Android. There is no device that eat less. More power, more they need, less power less they eat. Simply as that.
At the moment I don't know specifically that they are used for so maybe an expert can give you a better explanation.
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My Evolution: Sony Xperia Z1 - UB + 4.3 Rooted + Deodexed, HTC HD2, HTC Diamond 2, HTC HD Blackstone, Nokia 7070 Prism, Qtek 9100, Nokia 3100, Motorola T720i, Nokia 3310, Sony Ericson PF 768 (Blue)
How Get Root on Sony Xperia Z1 with Locked Bootloader and the Latest Android 4.4