1
The device has 8GB of memory, however this will be partitioned for system, apps (data), recovery, bootloader, cache and possibly other sections, before you get the 'SDCard' partition you can access.
There may even be a disclaimer somewhere in the manual/on the box that states that you won't get the full 8GB of space after formatting and system files.
It's expected and I'm afraid there's very little you can do about it.
Edit: Found a link to the device, note the following:
*2: ‧Some storage capacity has been set aside for the operating system and pre-installed apps. Actual usable storage space may differ from advertised storage.
thanks. Really, android 5 takes about 5 GB. I will change my tablet to 16 GB – Andrei – 2016-01-08T14:17:40.133