Usually, in case of any media used for data storage, you will not be able to use the entire space because the media needs a little space for drivers and other services. However, these files should be hidden. In this case, I recommend you to download Malwarebytes and perform a complete scan of your memory stick. The files that you mentioned might be created by a virus or malware. If the scanning process doesn't reveal anything, the files are needed by the memory card and there will be nothing that you can do.