My macbook battery is fully charged on menu bar but on Coconut Battery says charging Here is my screenshot when my fully charged but still charging:

After a few minutes not charging again:

I'm just wondering, will my MacBook overcharge or over discharge or this is normal behavior?
Why design capacity is not consistent, sometimes 74%, 75%, 80%, etc.
Everything seems normal to me.
Batteries are much more complex than most people think. There are many factors such as heat/temperature, actual charge, power draw, lifecycle/age,... that contribute into &quot;inconsistent&quot; measurements.

Why design capacity is not consistent, sometimes 74%, 75%, 80%?

This is completely normal and every battery experiences this, especially as they get older. This again depends on many factors and the number will decrease over time/cycles.

I'm just wondering, will my MacBook overcharge or over discharge or this is normal behavior?

When at 100% your battery will stop charging. It will start charging again, usually a couple percent below 100% if continually connected to power. macOS tries to smoothen the curve, i.e. it makes you think its constantly at 100%, which it isn't. This behaviour is normal.
There are many excellent articles that can explain the physical details much better than I can, but there isn't really anything you should/shouldn't do with your MacBook in terms of battery, except trying to not (over)heat it. So just use your MacBook, but don't put it in bright sunlight for example.