probably weighing some poop. I always try to weigh after poop. But even still the weight can fluctuate a bit. A good poop can weigh 20 grams, especially if there is some "Holdover Poo" that didn't all come out last time. LOL, we snake owners love to talk about Poo.
Also, didn't Loopy recently shed? Did he skip a feeding during that time? Did you account for the weight of the shed no longer being attached to him. It's not much but can be a couple of grams.
There is a lot that can affect a snake's weight. And to answer your question, Yes, Rufus goes up and down a bit in weight too. But it's mostly up. It's kind of like watching the stock market. There will be fluctuations along the way, but as long as the "Trend" is upwards, it's ok.
It's when they start losing weight and Keep losing it that you should be concerned.
And, very nice graph Gail. I wish my graph making skills were that good. I keep a handwritten journal on Rufus, his weight, growth, sheds, poops, feeding, etc... it's all there, I just don't have the computer skills to make it a nice graph.