Windows 8 measures the memory usage in a funny way anyway, I think it compares how much memory each process uses in comparison to the total, so...
The PC could have 256GB RAM, but if only 3GB is being used at the moment and 2GB is being used by one app in particular, it'll say "66%" next to that particular app.
At least I think that's how it works, I haven't got a Windows 8 machine in front of me at the moment!