Drives, whether hard drives or USB drives, have space overhead when they are formated. There are unused bytes between the segments and clusters and bytes used for the File Allocation Table and bytes held in reserve to recover from damaged bytes as they occur over time.
The overview image at the top of this link shows a rough idea of how a formated drive looks. But it doesn’t show the slack bytes between the custers.
Oh, there are indeed 16G bytes on the device. They just can’t all be used for pc files directly. There’s overhead involved in making the bytes into a pc format.