6.5GB of internal storage is missing, how can I free it
The usual question. According to GSMArena, your device has 8 GB internal storage altogether – which is not only the /data partition (where your apps and user data are stored), but also includes the /system partition where the OS resides. Depending on manufacturer and device, the latter can consume up to 8 GB of your internal storage chip (usually devices with heavily modified GUIs such as Samsung's TouchWiz are the ones using the biggest sizes for that).

Though on a device with 8 GB altogether 6.5 GB for the /system partition sounds a bit much, that's the part excluded from your listings. To check how much is consumed by /system you can use a terminal app (or adb shell) to run the mount or df commands (the latter gives easier to read output sufficient here).

In addition to the two mentioned partitions, there're also other partitions consuming some space (e.g. /boot and /recovery), but they're usually rather small (much less than 100M in most cases) while not to be ignored in this context. For a full list, you can e.g. check with this XDA post (which unfortunately does not include sizes).

A more detailed discussion (also on the HTC Desire 510) can also be found at AndroidCentral, and give you additional clues.