Storage Problem!

Because you never had 16GB's to begin with. The answer has to do with electronic devices and their operating systems using a different definition of "gigabyte" ("GB") than that of the manufacturer's of storage devices such as memory cards and hard drives. When manufacturers of such devices say "GB", they mean what is called a "decimal gigabyte", which equals exactly 1,000,000,000 (one billion) bytes (a byte is a fairly basic unit of storage in computers). When your device's operating system describes the capacity of the storage medium, they typically use what is called a "binary gigabyte", which equals exactly 1,073,741,824 bytes. Because of this discrepancy, a device has less binary gigabytes than decimal gigabytes. It turns out that a decimal gigabyte is approximately 93% the size of a binary gigabyte, so if your iPhone is 16 decimal gigabytes, your operating system will report that it is approximately: 16 decimal gigabytes * 93% = 16 * 0.93 = 14.8 binary gigabytes. Subtract for the iOS & you have 12 ~ 13GB's or so.

Apple doesn't do trade ins.