The name brand models often make it very hard to upgrade. Sometimes they combine one or more components such as a sound card and modem so that to put in a new sound card you have to replace the modem as well. Or even worse, they build the sound card right into the motherboard, so that you have to replace the whole motherboard. However, the motherboard is specialized so that only that specific motherboard can be used in that computer case. And the software they give is also often specialized.
Clones just require a simple upgrade.
In Clones the upgradeability is at a max and the components can be mixed and matched. The operating system will work no matter what components you choose to add or upgrade, since the Windows operating system is not customized by the computer maker.