They usually have a margin of plus or minus 5%. That being said, mine used to read 29% and I knew that wasn't right for what I weighed and my muscle mass. So, I looked through the directions and there is an athlete mode on mine. Basically, it said if you exercised more than so many hours a week, you should switch it to athlete mode. I did that and the reading said 21%. That was much more on target for what I thought it should be. So, you may want to read your manual, especially if you exercise alot.
PS, if you own the handheld one by Omron, like I do, that also has an athlete mode.