That's because you have one set of Older IDE cables vs the newer ones. The newer ones have like 80wires vs the older ones having 40. The newer thinner ones with more wires still work for both HD's and CD-Roms, you just happen to have one of each, doesn't mean that one is specifically for HD and one is for CD-ROM because you could flip them around and still work
The newer 80 connector cables (the one with thinner cables) contains just that- 80 connectors. Between every data line, there is a thin line going no where- this is a ground wire. Its purpuse is simple- when electricity flows through a cable, it emits an electrical field around that said cable. This field can alter the field of other cables, or flat out cause a current to flow through anoth wire (Some amplifiers work in thsi manner). Hwn moving data at high speeds, it si common for these electrical/magnetic fields to alter and distort the data of surrounding cables. To remedy this , a ground cable is placed inbetween to act as a buffer and increase data performance at ATA100/133 speeds.