Why defrag does not move some clusters?

The Windows version of Norton Utilities was originally released for Microsoft Windows 95. Norton was in need of a major update as the VFAT system used by Windows 95 to provide long file names was incompatible with utilities such as Norton Utilities' 'SPEEDISK.EXE'. However, the Windows version, called 'Speed Disk' works with much greater speed than Microsoft's supplied defrag program, as it moved groups of clusters, not single clusters as Microsoft's Defrag. Changes in the way Windows operates meant that many of the old utilities were either dropped completely or replaced with GUI based versions. However, with the advent of Windows XP onwards, Norton's 'Speed Disk' reverted back to single-cluster defragmentation.