Is it not a little bit stupid, to first defragment your disk with slow optimize and to disrupt this with fast update? And the comments stated that these belong together. Daily use fastupdate and once a month use slow optimize.
As Darlis said, FastUpdate does not create fragments, so what you observed must be something else. The FastUpdate script basically only fills gaps, and makes sure all the files are in the right zone. It does not produce a fully optimized disk, like SlowOptimize does, and that's why you need to run SlowOptimize again once per month or so.