Is it due to dual platter design or something?
Fragmentation occurs at the filesystem (logical disk) level, so the physical storage hardware does not play a role.
1% is nothing to worry about. My guess is that it's just a reporting error by the Windows defragger. It does not report or defrag larger fragments (64MB or bigger, supposedly there's no benefit from defragging these) by default which is just stupid if you ask me. It also does not defrag some system files.
Just download the free evaluation version of a good third party defrag utility and analyze with it. It will also show you the disk map. Usually no two defraggers will give the exact same analysis because each one differs in it's interpretation of what's fragmented, but the numbers should be similar.