It means you are utilizing the full power of CPU. I personally feel monitor never give 100% accurate readings. It only shows approximate results.
Normally this happens if you are doing things like - copying over 2 GB size files, Running apache, sql, xsp, etc servers side by side, editing or rendering videos, writing DVD disks etc.
Playing any graphics game means video editing on the way. You are continuously editing the images. Thats why it shows such high cpu usage. Its normal. Any graphics related application takes huge ram and lots of processor cycles to run.
May be different OSs calculates and shows CPU usage differently. May be mac shows processor usage by processes not as a whole. I only used windows and Ubuntu.