Not sure if this is problem, as if the disk drives are related/combined somehow, then it is completely normal and expected.
Is it possible that drive 5 and drive 6 are in RAID configuration or a hard disk cached by an SSD?
Generally Hard Disk Sentinel follows the order of disk drives you see in Windows Disk Management, exactly to allow easier identification.
There is an exception: when the Disk Management shows a RAID array (or a similar virtual drive, for example a hard disk + SSD cache, which appears as a single drive in Windows Disk Management). Then Hard Disk Sentinel (as we'd expect) attempts to detect the physical disk drives behind this virtual device and shows them separated - of course with the same logical drives assigned to them, to show that these are managed together.
If you use Report menu -> Send test report to developer option, I'd be more than happy to examine, check the actual raw response provided by the disk (RAID) controller and verify the situation. If there may be any issue, the developer report helps to check what happened and what to do to improve the situation.