How accurate are the scales that scan your body for your mussel mass and your fat mass?

I have been using a Tanita Scale. The good news is that I have lost 13.5 lbs over the past month, but the disturbing news is according to the scale I have lost 5 lbs from fat mass and 8.5 lbs from mussel mass.

I have been going to a nutritionist; she is helping me with my diet. I have been watching my diet and working out like a dog both riding and weight training.

My nutritionist tells me not to worry about it, that it probably something to do with water weight, but it does concern me.

Should it? How accurate are these scales in determining body fat percentage? I understand they use electrical current to scan the body.

thanks