Under certain conditions, Sync clients will re-download files even though the files themselves have not updated. This occurs because of
SYNC-1704, where changes to a role/permissions will update the modified time of SyncDLObjects.
If the SyncDLObjects have checksums (which occurs by default when files are uploaded through Sync and are under 1gb), the files will not re-download since the checksums will match. However, if there are no checksums (uploaded through the web UI, webdav, etc), then these files will re-download.