firstly, please specify your current spec, but its mostly because the other 1 GB's is used as VRAM by iGPU, if you you are using PC then you can configure it, but if you are using laptop, then I'm afraid you can't, at least on most models.
EDIT: also,I don't think having only 7 GB (or 6.9 GB) will affect much, since imo, there's still no game out there (yet) that really need 8 GB of free RAM, as long as you don't multi task.