The balanced characteristics of U/UTP (unshielded) cables are not only determined by the quality of the components themselves (eg twisted pairs), but are also affected by the surrounding environment. Because metal around U/UTP (unshielded), hidden “ground”, pulling during construction, bending, etc. will destroy its balance characteristics, thereby reducing EMC performance.
So, there is only one solution to get lasting balanced characteristics: ground all cores with an extra layer of aluminum foil. Aluminum foil adds protection to the fragile twisted-pair cores, while artificially creating a balanced environment for U/UTP (unshielded) cables. Thus forming what we now call a shielded cable.