So I ran an analysis today, saw a fragmentation of about 23%, ran a defrag to completion (0% fragmentation), then immediately did another analysis and saw a fragmentation of 12%. Well 12% != 0% so what exactly is going on? Re-defragging doesn't seem to change the numbers all that much, but it always ends with a 0% yet an immediate re-analysis will yeild a result greater than 0%. So I'm assuming I'm not actually degfragging anything...
or you could, rightly so, assume one of these many things:
1) you are using windows vista/7/8 and have not turned off VSS while Defraging http://forum.piriform.com/index.php?showtopic=35459
2) your computer is in use and thus is already begun to fragment things (perhaps your Antivirus was active during the process, or perhaps you were surfing the web during the defrag, or perhaps windows/java/flash/realplayer/etc update ran whilst Defraggler worked.
3) perhaps the scan is finding locked files it is unable to defragment (such as antivirus definitions, an open document, or better yet restore points and hibernation files)
4) you are making a molehill out of a mountain and will notice little, if any, performance difference between a drive with 12% and a drive with 0% fragmentation in a infinitesimal snapshot of a scan, when a hard drive is in the constant act of fragmenting.