Say you're using your laptop in your bedroom, on AC, battery charged to 100% and you want to take it to the kitchen.
so you do, and walk across the house and plug in again in the kitchen.
Do you really want the laptop to put wear on the battery by charging it that tiny percentage that was lost during that walk? Of course not, it would be silly.
This behavior protects the battery against excessive topping off charging. It also helps prevent the batteries from being inadvertently overvolted.