Macbook Incorrect battery percentage reading

Two things come to mind where this 10% increase may be seen.

1. The information on the login screen is not always accurate (seems it may have improved with Sonoma, but I'm not certain by how much). Only check the battery charge level once you have logged into macOS.
2. By the time you log into macOS and can read the battery charge level, the laptop could have easily charged the battery by multiple percentage points since these more recent Macs charge their batteries extremely fast.

Plus it is possible for the battery charge level to increase by 1%-2% even without the power adapter connected.

While most Lithium batteries will last 3 - 5 years, some may wear out or fail sooner since it depends on many factors (battery itself, how laptop/battery is used, how laptop & battery are handled....impacts, or used in extreme heat, and whether the battery is left it 0% charge for extended periods...or even 100%).

As long at the battery is performing for the time period you need to use the laptop, then just ignore the battery and battery information. If the battery performance is no longer sufficient for you usage, then you need to investigate by checking the battery condition in macOS for "Service Recommended" condition, otherwise look at the software & system configuration in case the system is working harder than necessary & draining the battery more quickly. Some battery failures cannot be easily detected (I developed a method for better identifying borderline batteries for my organization....actually still trying to improve the method for the M-series Macs since they have changed so much), so this is why I recommend this method.

You should not be frequently running your battery down to the 10 percent level.

That computer is a battery-CAPABLE device. It is not optimized as a battery-operated device. (It is NOT an iPhone.)

Your computer performs best when connected to AC power, such as the power adapter. It can use the full output of the Power Adapter AND when doing especially challenging work will also freely "borrow" power from the battery. In some cases, even with the power adapter connected, the charged state may decline during very stressful work.

When used only on battery, your computer has no extra cushion of power, and may perform more slowly. However, for ordinary non-stressful tasks this may not be objectionable (possibly not even noticeable.)

In general, you should ALWAYS connect a power source when it is possible to do so, and only run on batteries (which could be somewhat slower) when no power sources are at hand. Your Mac will NEVER over-charge. 

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Of course, this computer is YOUR tool, and you should use it the way it works for you. But you could be getting better performance.

Hi,

I think battery level meter is not that accurate so that 10% level jump to 20% when you connect charger.

Have you checked the battery health status? Sounds like phantom charge