There is no problem with your pendrive. It's just how the PC and pendrive populate the bytes information.
The manufacturers of storage devices set the 1000 bytes = 1K, so like normal number conventions 16GB = 16000000000 bytes.
On the other hand, PC calculates 1KB as 1024 Bytes, therefore 1GB = 1024MB. If you make the calculations 16 GB read by 1024 system makes 16000000000 / 1024 / 1024 / 1024 gives you roughly around 14.9GB
Hope that I helped.