Hi. The downloaded vmlinuz-bionic, vmlinuz-bionic.md5.txt, dCore-bionic.gz, dCore-bionic.gz.md5.txt files (if dCore-bionic is being used) are kept in that directory in the case dCore-usbinstall is run again as to not have to redownload those files next dCore-usbinstall session. Especially if the TCE directory is persistent. The md5sum is checked on those files on the next run of dCore-usbinstall, and new files are downloaded only if the md5sums don't match. Saves time and bandwidth. It is by design.
Thanks.