As I am sure that you are aware, the right side of the heart pumps blood to the lungs at low pressure so the blood can be re-oxygenated, and the left side pumps the oxygenated blood from the lungs at high pressure throughout the rest of the body.  In looking at the heart, you can see that the right atrium is much smaller compared to the right ventricle as opposed to the left atrium to the left ventricle.  Think about it, if you push a smaller amount of fluid into a larger area, you have less pressure than if you push a large amount of fluid into a smaller area.  And this is explains why there is a tricuspid between the right atrium and ventricle and a bicuspid between the left atrium and ventricle.

You see, the left atrium and ventricle are pushing blood out at very high pressure (the systolic number on a blood pressure test).  By the time it reaches the right atrium, it is filling the right atrium at the low pressure between heart beats (the diastolic number on the blood pressure test).  Since the right side of the heart must move the same amount of blood as the left side but at much lower pressure, it is necessary that the valve open much more readily to allow the same volume to pass through (the three parts of the valve would allow it to open more circular).  Conversely, a bicuspid valve works better on the high pressure side because with only two sides to the valve, the muscles and ligaments are able to cover more area and be better able to spring back from the high pressure on the left side of the heart.
Well, when you actually look at the valves, there are not distinct segmented cusps, like in the aortic and pulmonic valves, where normally there are three equal cusps each. The valve leaflets in the mitral and tricuspid valves are broader flaps of tissue, anchored to the distal ventricular endothelium (protruding masses of myocardium called papillary muscles) by cords (chordae tendonae). While the left ventricle is more muscular, it is less flexible, or expandable, and the annulus of the mitral valve more stable. Two *leaflets* are sufficient to effectively close the opening back to the left atrium. The right ventricle is thinner, more potentially expandable, and the *leaflets* less well defined. It is indeed difficult to specifically tell where the three leaflets start and end.

The terminology we use is from ancient days, and in some cases, such as this, can lead to misconceptions.