A PV module nameplate is determined by measuring its DC output under a 1000 W burst of light with spectrum similar to solar irradiance. For example, if a test measure 322 W under a 1000W flash, the module nameplate power will be 320Wp. See Module Quality.
Under these test conditions (a brief first illumination), a phenomena called ‘Light Soaking’ occurs whereby the efficiency of the solar cell increases by a few percent. It means that the test overestimates the module power by a few percent. In real operation and continuous sunlight, light soaking does not occur and the module is less efficient than indicated in the nameplate.
For that reason, modules time derating typically starts by -2% on day one. If a manufacturer derating warranty is -20% efficiency over 25 year, it will be -2% on day 1 and -18% over the remaining 25 years.