Different free space determining...

v1.10

In drivelist it shows:

   Disk   |  File System | Capacity |	  Used	  |  Free Space  | Fragmentation | Status
--------------------------------------------------------------------------------------------
Work (D:) |	 NTFS	 | 114,9 GB | 108,4 GB (94%) |  6,5 GB (6%) |	Unknown	|  Ready

But when i press Defrag i get warning "You have less than 5% real free space..." I think it's a little bit confusing.