It's interesting that it weighed you LESS in the evening.
Mine is accurate exactly within two pounds. It weighs me two pounds heavier than my actual scale and it's extremely consistent. I figure I could go with either the Wii fit weight or my scale's weight. As long as they are both consistent, it doesn't really matter.
What I would do is get a weight from a calibrated scale, such as the doctor's. School nurses have great scales too if your local school nurse wouldn't mind weighing you.
Then compare that number to your scale and the Wii. That will determine what is accurate. Five pounds would bother me too. Two pounds does not.