You don't really 'gain' any free space from defragging; a defrag only rearranges file fragments to make them contiguous (well, you may gain tiny bit of space if the drive was highly fragmented initially and post-defrag free space consolidation was very good).
As to your problem, it's quite puzzling but seems to be not uncommon. I really don't know the solution, but am thinking out loud: I used to think a loss in disk space after running the native defrag utilities in Vista and Windows 7 arose from the incompatibility between the native defragger and the shadow copy service (although system restore is disabled on E volume, I *think* VSS may still be active since you have SR enabled for C..I am not at all sure of this). However, see this post here by GeneO Strange things happening on w7
that states that the vista/7 defraggers are VSS-aware. I suspect
this compatibility is still not up to the mark, and the windows defragger is what is causing the (temporary) loss of space.
I am not certain of any of the above stuff, so take it FWIW