Actual capacity does in fact differ from the stated capacity of a USB drive. When you are supplying data to be uploaded to a USB drive, it's best to leave at least 10% of blank space to ensure that none of your data gets cut off.
For example, on a 1GB flash drive, we recommend supplying no more than 900MB of data to be uploaded. Due to rounding by manufacturers and a whole lot of fun computer terminology - we need to be aware that not all sticks carry their stated capacity.
Technology presumes there's just one right way to do things and there never is. ~Robert M. Pirsig