There are several factors at work here.
First up, as previously noted, some of your device's 16GB is taken up by iOS and the bundled Apple apps that come with the device (Contacts, Safari, Mail, etc.), so the amount of available space on even a freshly initialised device will be less than the advertised capacity. This most likely accounts for the majority of your "missing" GBs.
The second factor is that for iOS devices the advertised capacity is quoted in decimal gigabytes, i.e. 1GB = 1000^3 or 1,000,000,000 bytes, whereas the operating system always reports capacity in binary gigabytes, where 1GB = 1024^3 or 1,073,741,824 bytes. Thus an iPad that has 16,000,000,000 (16GB decimal) bytes of storage will be reported as having only 14.90 GB of capacity by iOS. Same number of bytes but different number of GB depending on whether you use decimal GB or binary GB. This is not unique to iOS devices, Windows PCs and Android devices work like this too. Macs used to work this way but several releases of OS X ago Apple switched the software to always report capacities in decimal GBs instead. Unfortunately they have yet to do this for iOS.
And then there is a final factor that affects how many usable GBs you have on any device or hard drive - formatting. Every storage device has to be formatted before it can store data files and the formatting process uses some of the device's storage (the exact amount varies depending on the OS and filesystem in use, it gets quite technical). The storage capacity for iOS devices (like PCs and Macs also) is advertised as unformatted, some quantity of that number will be used by the formatting process and is thus unavailable to the user. This will be the smallest contributor to your "missing GBs" issue, but it will make a small difference in addition to the previously described factors.