These are commonly called shielded cables because they are made specifically to protect the data they carry from outside interference. The two wires in each pair are then twisted together. This twisting improves signal integrity compared to running all the wires in parallel to each other. The outer layer of the cable contains the data and other signals. The purpose of this three-layer structure is to further insulate the most important signals to improve data integrity.
External cables have a round cross-section, reflecting the circular layers mentioned just above.
For internal cables all these special steps are not required to protect the data in the wires from external interference. These are much cheaper than external cables to make. Close-up view of an external SCSI cable. Inc. Image used with permission. An assortment of different internal ribbon cables used for connecting SCSI hardware.