charge cycle 13 and battery is down to 99%

You need to wait longer to get a more significant reading. For instance my 2019 MacBook Pro was at 92% after 16 months, 91% after 19 months, and 92% after 25 months. I don't think these numbers are so precise that a 1% difference is significant. Batteries wear out over time, it is not just the cycle count; over time as the battery is in use, it will lose capacity.

Another computer, a much older MacBook Air, was at 88% after 47 months, 87% after 70 months, and 90% after 101 months.

The MacBook Pro has less than 50 cycles, the MacBook Air has 300 cycles.

Eventually these batteries all wear out and need to be replaced. It's an inexpensive service when it is needed. Set your Preferences on "optimize battery charging" and don't worry about it. (The MacBook Air is too old to even offer optimized charging.)

The difference between 99% and 100% is smaller than the uncertainty in those measurements.

Running a M1 MBA bought March 2021 and the battery Capacity does up and down all the time.

Use a Third Party Application just to keep a history of battery performances.

It, the application only reads the sensors on the computer, so no magic there.

coconutBattery 3 Paid for additional Features or free

Batteries are considered Consumable Products. Over time it will degrade to a point where it needs to be replaced. The usage is normal as the capacity will go up and down all the time and never stay always at 100%. Apple Batteries are rated for 1000 Full Battery Cycles and / or 80% Capacity before needing Evaluation or replacement.

For additional reading on Battery and Apple Computers this link may help. About battery health management in Mac notebooks and especially for the M1 and Big Sur  & Monterey If battery charging is paused or on hold on your Mac

Depending on the Age of the computer, some aspects from the above links, may or may not apply to your computer.

A feature introduced into Big Sur & Monterey was to preserve the number of Full Battery Cycles - there was an Algorithm coded into the macOS. It is perfectly Normal for Big Sur , Monterey to Pause / Suspend charging the battery beyond 80% and hold it at that level. One can nudge / over-ride this by clicking the Battery Icon on Desktop Top Bar and allow charging beyond 80%. From observations, it will Pause / Suspend again once reading 90%. Same procedure to over-ride and will charge to 100%

gee71 wrote:

I have a charge cycle of only 13 on my m1 MacBook Air but my battery is down to 99% health already??????

ref:

Why is the battery capacity declining rap… - Apple Community

Were you expecting it to increase from 100%?