I think it is because of the battery settings. There's 2 battery settings, maximized and optimum.For maximized, its like our usual usage of the battery without lenovo energy management. It will charge the battery until 100%.The next option is optimum where the main purpose of this function is to prolonged the lifetime of the battery. So, what this option do is maintain the battery at a certain level but not at 100%. If the battery level is less it will charge the battery but if the battery level is more than the required it will drained the battery. Hope this helps.