Martin
In task manager it lists processes and apps. It does not list things like shared video ram, fast cache, etc. For a more accurate view of ram usage you should use resource monitor. In task manager>performance tab>open resource monitor (or type resmon in search)
Basically Win 8.1 takes about 1/2 of the ram and puts it into fast cache (a good thing) so that when you launch an app it launches faster.