When you specify a drive letter to scan during manual scans, the OSCE agent always scans the system drive (usually C:) before the chosen drive letter (D:, E:, F:, etc.). This happens even though you do not select the system drive for scanning.
When a manual scan is triggered, the Damage Clean Engine (DCE) is will be called to inspect critical system paths such as …\System32, …\WINDOWS, etc. for malware.
By design, this is the intended scan flow.