@trium,
I'm sure there's an explanation but it doesn't matter much to users who usually want an app that works. Not an app with issues and reasons why something's wrong.
I'm facing the same issue with 3 HDD external 4TB and 5TB hard drives
Windows defrag, O&O, and a few other disk defragmenter I tried all see my HDD drives as HDD. Only Defraggler sees them as SSD...
Doesn't matter much as the optimization runs anyway.