When your new laptop battery is delivered, the battery will not be fully charged. We recommend that you charge the battery for at least 16-24 hours continuously, or even longer. We also recommend that you do this for the first 3-4 charges as well.
Yes, this is very frustrating, and will stop you trying your new purchase out, but if you do follow the instructions. It can make a huge difference to the performance you'll get from the battery in the future.
Note that the laptop will indicate that the battery is fully charged after a few hours, but for the initial charges, you should ignore this.
A battery is made of several cells wired in series. Although good quality batteries are made with matched cells, there will be some variation between the cells in any battery.
When you charge the battery, some of the cells will be fully charged first. The charge current has to pass through all of the cells, and you have to be sure that the trickle-charging completely fills every cell, even though some of the cells are "full" and therefore the charger control circuits are cutting back the charge current. The way to ensure this is to leave the battery on charge for a long, long time for the first few charges.