Interesting thread, and a good question. It boils down to whether you are gaining lean tissue (including increased bone density) or fat. If the latter, I would suspect carb creep, especially if there are any processed foods in your diet. (Sometimes manufacturers re-formulate with added sugar, etc. and it is also easy to consume a bit more carbohyddrate than we realize.)
I’ve been keto for two years, now, and I notice that my body composition changes all the time. My weight has been stable since late 2017, but my ability to wear certain trousers seems to vary. The occasional yielding to carb cravings is probably a factor in my case.
The phrase “converting the calories differently” was what attracted my attention. We can’t add muscle from carbohydrate or fat, that requires nitrogen, which means amino acids, hence protein. One of the reasons we require a certain minimum amount of protein every day is that we are constantly losing a certain irreducible amount of nitrogen every day, from the deamination of amino acids (the nitrogen gets converted into uric acid, then to urea, and is then excreted in the urine). Autophagy of old tissue can help recycle nitrogen, of course.
But it is entirely possible that if your protein intake went up, and if it included enough of the essential branched-chain amino acids (leucine, iso-leucine, and valine), that you actually did increase your muscle mass. And don’t forget about the possibility of increased bone density—what I learned from these forums is that although we think of bone as being made of calcium, it is actually a protein matrix that holds the calcium in place.
