I lost 2.3 GB after unjailbreak my iPhone 4s
Whether you're JailBroken or not, and regardless of if you've ever been, you'll never actually see the full 16GB on your iPhone. Part of that 0.31GB of "Other" that iTunes reports are the remnants of the JailBreak. Restore iOS again, and then "Set up as a new phone" when going through the activation process. You'll never gain back that 2.3GB, because that's what iOS is using to store the operating system. To be more detailed, the 13.62GB of capacity is only the capacity of the User's partition on /var/mobile/. The other 2.3GB is given to / (the System partition).