I think your logic is forgetting about the digestive system. Do snakes not burn calories or take nutrients from the food? Or dissolve some of that weight in the stomach acid?
There's no chance a snake will gain 10-15% of its body weight weekly. I'm new to snakes but it just doesn't make sense.
I weigh mine after they shed (sheds are normally accompanied by dump as well) I wouldn't worry too much about getting an exact weight, +/- 5 grams isn't going to matter in the long run