This is because the hardware manufacturers always assume the maximum battery life of your laptop. After a particular span of time or after the battery has been recharged for a specific number of time, they expect that the battery should be replaced anyways.
If your laptop is 2-3 years old, it is likely that your battery's life has reached its maximum time or number of recharging limit and therefore you are displayed with this message. However if you have been using your laptop wisely (as it seems true in your case) and the battery health remains intact, the operating system detects the health status accordingly and displays it to you.
The bottom line is, if you are not experiencing any battery backup issues and your laptop can run on battery for long hours, you can ignore the message.