Hi Melissa,
Let me give you my two cents' worth:
A bell curve is a theoretical distribution of scores across a given population. Applied to PMS, this means that the performance of the majority of a given population will tend to gravitate towards the center, i.e., average, while the extremes would comprise either the poor performers or excellent ones. This simply means that you can't have 100% stars in a team; there have to be the so-so (average) guys and low performers. The curve is mostly used as a control mechanism.
Hope this helps.
Cesar