The many models of Android smartphones and tablets on the market. Because device makers are free to enhance the Android operating system with user interface additions, and because vendors do not update all their customers' devices with the latest version of the Android OS at the same time, there is a large number of Android hardware/software combinations on the market. Not only is it confusing for users, but it is also difficult for developers to ensure that their apps work on every device. See
Android and
ChromeOS.