If you removed the power cord from the modem, the status is reporting what it was the last time it was plugged in.

If you plug the modem back in, it will update the usage to what it's supposed to be at the current time.

If you did not remove power to the modem, is it possible something or someone was using your wifi without your knowing it? For example: Several devices such as cable/satellite TV boxes could have used your wifi for simple things like software and guide updates.