I presume you are using an SSD. The way that SSD's manage storage read & writes require them to have a portion of the drive set aside for managing retrieval requests when your PC wants to use them. Most are defaulted to 10% of the drive's total capacity but it can be up to as much as 25%.
EDIT: The technical name for it should you wish to do more research is "provisioning" and depending on your SSD and accompanying software you may be able to adjust the amount of the SSD that is "provisioned", but i would not recommend going any lower than 10% or you start to lose some of the benefit of the SSD.