Yes, the percentage charge remaining will change, even though very little discharge takes place, such as when you have it off. It's a computed estimate, which is the percent charge remaining if this same discharge rate continues (more or less)
I do prefer the percentage, to "time" remaining, as that can vary wildly.
Your small change is very normal.
Just plug in, let it charge up while it is off. Take advantage of that idle time to keep the battery up at a higher charge.
Finally, try not to be concerned, unless the actual use time that you get is very short. IMHO, less than 2.5 hours battery use from a full charge would be cause for a question.
I very rarely turn my Macbook off and leave it off - maybe two or three times a year. It's a portable, so I use it as a portable - no reason to have it OFF. It gets used, then it sleeps 😀
