Some ear buds models (like the CX 300-II) feature cables where one side of the cable is longer than the other. These are called "asymmetrical" cables and is a deliberate design by Sennheiser.
The longer side of the cable goes behind the neck to the other side of the head and into the ear so instead of having a two cables hang in front of a user in a "Y" configuration an asymmetrical configuration only has one single cable that is off to the side where it is out of the way. It is recommended to route the cable above the ear and then down into the ear canal. The cable exiting the ear bud should be pointing up.