The manufacturer may have his own reason[1] to do some over-provisioning, as a user I intentionally leave some ssd space unpartitioned to avoid the terrible performance drop when the SSD gets full.

[1] Remember this microSD article ? http://www.bunniestudios.com/blog/?p=918 I wouldn't be surprised to learn that manufacturers combine controller and over-provisioning as a cheaper and more flexible alternative to a full test of the SSD.