Yep this has to do with glycogen/water weight storage. The body doesn’t just store energy as fat, it also basically squirrels away sugar water for itself. Until you’ve depleted those stores the body is holding on to a bunch of extra water. Excess carbs are an especial culprit for this.
What do your off days look like? Are you eating slightly over your calorie goals, or 2 times your calorie goals?
If you are eating heavy on the calories then it would be reasonable for your weight to not go down.