[dragonwriter]

While input-side variability may be a factor, if you are doing time-and-exercise-description without closer monitoring to determine your calories out, output-side variability is also a likely factor here.

Also, doing that much exercise, you can maintain a calorie deficit (a real deficit of calories absorbed vs. burned) and not lose weight by gaining muscle as fast as you are losing fat (muscle has a lower stored-energy content than fat); you'd need more than just stepping on a scale to distinguish that.