My weight goes up and down by some 10% every couple of years, I cycle and run (ultramaraphons) and climb and more, and I track and analyse both food and spending with common tools and myself. Which is why I am acutely aware of how at least my body behaves in this respect. And I see people around me who do similar things.
Your point seems to be based on the idea that if it is 5%, it is the same as zero, because metabolism compensates (?). I do not know if this is the case at all or if this is relevant enough to change this 5% number. If this is the case, it is a factor, but it is something peculiar.
Instead, I find, that while a single 10hr trail run spends days worth energy of usual activities, several 5% factors each day, which grow from habits like brooming or taking a walk instead of taking a bus, quickly exceed, or at least strongly contribute to, extreme individual spendings. Also while a long event seems to cause immediate weight loss, it is almost entirely water. So it is a bit hard for me to believe these small spendings are zero. In fact, I find that people often underestimate how simple habits change weekly calorie spending. At least for me, these things make much of a difference in the weight change.
And yes, I have some brooms, and I broom for some 15min a day probably, plus maybe 1h per week.
Inertial systems are all equal in a certain relevant here sense, if there is no need for account for your movement relative to Sun, Galaxy, CMB, or anything else. Yes, in this sense, Sun also rotates around Earth.