It can be difficult to gain muscle mass and lose weight at the same time. Usually, what ends up happening to me is that some of the fat gets converted into muscle mass. Don't be too surprised at first if you gain some weight. Typically, the weight you gain in that case is probably going to be extra muscle mass gained from working out.

The scale you were using in the gym is I think is one based on electrical impedance which means it just sends a current through your body to determine your body mass composition. It's somewhat inaccurate compared to other more expensive methods but it gives a decent ball park figure.