How sophisticated is Defraggler?

I've used Windows Defrag, of course, but noticed it does not compact the free space between chunks in the display window. In fact, it creates more gaps than it closes, sometimes, during the defragging process. Norton Speedisk, on the other hand, carefully maps out the desired optimum disk layout, and squeezes everything together in an optimum position for max disk access speed.

Defraggler appears to be somewhere between the two in sophistication. It defragments the files, and apparently tries to consolidate free space, but what is its objective in "defragging freespace?" And why does that take so bloody long, compared to defragging the files?

Does anyone know, or care to present. the defragging algorithm(s) of Defraggler? Thanks!