It is normal for the used space on your drive to grow and shrink dynamically, Windows automatically manages some space on the drive like the Virtual Memory, Restore Points, and temporary space used for Windows updates, that space may be cleared automatically by Windows.
For the files stored in the C:\Windows\SoftwareDistribution\Download, do not attempt to change or delete any files or folders in that folder, that contains the installers for apps and drivers installed on your PC, if you delete data from that folder, you will not be able to uninstall apps or automatically repair or reinstall apps or drivers that may experience problems in the future.