(I'm no doctor but) I struggle to imagine that bone density can change within a day. For that matter I'd be shocked if actual muscle mass changed; I'd personally wager that its almost exclusively water weight (if nothing else, more blood getting allocated as your body does maintenance).
I'm no doctor either, so the below is mostly guessing.
I imagine bone density as well as any other biometrics change constantly. How much over the course of however long, I don't know.
I weigh myself daily on a scale with bioimpedance. I understand that this is not the most accurate way to measure body composition but it's something. My bone density is different every time I weigh myself, and it has some correlation with diet, exercise and water intake. Again, how much this is accurate or reliable, I don't know.
> I'd be shocked if actual muscle mass changed; I'd personally wager that its almost exclusively water weight
Given that a huge percentage of cell mass is water, you're likely correct. That means muscles get bigger and better hydrated. That should in turn make them stronger and more resilient. That's muscle building. The other bits (more organelles, etc) may or may not take longer to follow (again, not a doctor, not a biologist) - I have no idea.
In any case, the "study" more or less guesses that more exercise means weight gain due to reduced basal energy expenditure. That may be the case, but that's fine. Weight gain due to muscle mass is desirable.
The problem is that the phrase "weight gain" is normally used in contexts where it means fat gain, which is undesirable and dangerous for one's health.
