I think you mean BIA, Bioelectric Impedance Analysis, which sends a small current of electric waves through your body to measure percent body fat. BMI is an equation you punch in on paper using your age, height and weight.
To answer your question, yes. What causes that variability is the amount of water in your system. BIA's sends a small jolt of electricity from one touch point to the other. The more bodyfat you have, the more impedance it causes the rate of travel of that electric pulse. The rate of travel of the electric pulse through your body will result in your bodyfat percentage reading. The slower the rate of travel the more adipose tissue you have, the faster that rate of travel the lower your adipose tissue reading.
However, the more water you have in your system, the faster that electric impulse will travel (as water acts as a conductor) from touch point to touch point. The more water you have in your system, the lower that percentage reading will be. Therefore, The drier you are, the more accurate your reading will be.
Due to the hydration factor, variability of bodyfat by 2 to 3 percentage is very common. A 12 hour dehydration period will give you a more accurate reading. Easiest and safest way to go about that is to not drink water after 8:00pm and weigh yourself at 8:00am the next morning.
BIA bodyfat measurement is far superior to the BMI equation which doesn't account for muscle mass, but simply bodymass, which is null & void to someone who resistance trains. However, BIA instruments are either the hand held versions, where the electric pulse travels through your upper extremities, or they're the scale versions, where the electric pulse travels through your lower extremities. Your lower extremities are for the most part, naturally leaner and naturally contains more muscle mass, so you are likely to have a variable reading between the hand held version and the scale version, with the scale version generally giving you a more favorable reading.
With that said, regardless of the variability of an upper or lower extremity reading, a BIA score is still good for measuring progress. If you were 25% bodyfat, right or wrong, if it says 20% in two weeks, it's still quality information for tracking progress.
Take home message: The drier you are, the more accurate your reading. Whether that reading is the perfect number or not, you have a base to track your progress in the coming weeks and months.