Question about Synology Ds918+

I bought Synology DS918+ (4 x 8TB, WD Red) and I'm really happy with it, but what I don't understand is that I don't have 8 TB of storage per disk, but only 7 TB. Does that mean I'm missing a whole 4 TB ?!? Don't need a backup : Synology Hybrid Raid SHR without data protection Thanks in advance