Resting metabolism measurements are similar (in terms of fluctuation) to body temperature measurements, where you’ll find +/-10% change from the average.
While variation is inherent, to determine the most representative resting metabolism for your body, please review the recommended practices (see “How do I get a reproducible resting metabolism measurement?”).

Hour-by-hour, day-by-day, your metabolic rate reflects your body’s systemic adjustments, such as your liver’s processing of a macronutrient supply or your brain’s response to challenges.