Simply put, in technical terms, 1GB= 1024MB=1,073,741,824 bytes
But, storage companies put the GB defination like this, 1GB= 1,000,000,000 bytes (decimal system)
You lose about 73MB of space per GB that way. This rougly equals, 73X16=~1170MB of space advertised which doesn't exist in the drive.
So in a 16GB Flash drive, you get around 14830MB~14.8GB. Some of the space is used up for cache and encryption stuff and one usually get about 14.5GB free on a 16GB flash. Your PC is alright in showing that.