Most Lithium Batteries need to be replaced sometime around 3 - 5 years. Some may need replaced sooner, some later.
Plus the Maximum Capacity on Apple batteries actually fluctuates up & down.....a 5% difference is completely normal (perhaps even a bit more since I am being a bit conservative). So that 88% may be the lowest point and you may see it change to 93% again at some point (perhaps even a bit higher). People don't realize how much the Maximum Capacity of the Apple Lithium Batteries fluctuate since Apple only recently exposed this information to users.
If you use the Coconut Battery app like @Gl1tterUnic0rns posted and checked the "History" you would see these fluctuations.
Besides, unless the Battery Condition shows "Service Recommended" or the battery fails the Apple Diagnostics, then Apple won't do anything with your battery. When the battery's Maximum Capacity drops below 80% Apple will replace the battery (the battery condition at that time would show Service Recommended as well)....however, I think Apple has changed that threshold a couple years ago and may not be replacing batteries until the Maximum Capacity drops much lower....to around 75%-77%. Probably because the M-series Macs do run much longer on the battery than the Intel Macs (even when under heavy load).
Unless you want to observe how the battery behaves, I recommend people just ignore the battery information & condition until the battery no longer performs to your expectations. Then look at the Battery Condition to see if is is reporting "Service Recommended"....if so, then it is time to have the battery replaced. Otherwise ignore it.
All product advertising benchmarks are performed under very highly controlled conditions to maximize the numbers....numbers which cannot be achieved/matched with normal usage. It has always been this way. Compared to the Intel Macs, the M-series Macs are very good with their battery runtime due to the highly energy efficient customized ARM Apple Silicon M-series CPUs.
