1. Even if the deficit is small (200-300kcals), an extended period of this will usually result in a dip in thyroid and test numbers and, subsequently, BMR. As you lose weight, your BMR actually lowers since it'll require fewer calories to maintain your current weight.
2. Simple water retention from imbalanced electrolyte consumption could result in subcutaneous water. Each pint of water that you hold is ~1lb.