Lotta things could be going on. I wouldn't put money on muscle gain though, Keto diets keep your insulin levels pretty tanked, which isn't the best environment hormonally for hypertrophy. Besides, no one gains 1 lb of muscle per week unless they're loaded up on PEDs. I hate to get a bit graphic, but are you eating enough veggies for fiber? Could you be 'backed up?' My weight fluctuates 4 lbs during the day simply from water and food.