This is expected behavior. By default, Lansweeper does not delete any assets, even if they are removed from your network. If you replace a monitor with a new one, the old scanned monitor will still exist in the Lansweeper database. The newly scanned monitor will automatically be associated with the asset it is connected to if the monitor manufacturer and serial are stored in the client machine's registry and can be detected by Lansweeper.
Monitors will not count towards your licensed asset limit but in case you want to remove the old monitors from the Lansweeper database, you can either deleted them manually or use one of the
automated database cleanups under Configuration\Server Options.