Battery does not charge full time. If it did it would constantly generate heat & waste on your power bill and prematurely age the battery after it reached max charge level.
Charges up to its maximum capacity then charging stops & waits for the battery to drop down to some predefined level,, on mine its like 5 or 10% less than 100% before charging up to max capacity again. Actually my Lenovo is mostly on a/c and the battery is set for max life and it only charges to 50% -- then when it drops below 45% it charges back to 50%.
If battery has some age/use on it, it may not charge up to 100% capacity as it did when new.
If charge calibration is not perfect it may not ever show 100% capacity too.