It's a fact the mixed ability exists in all groups, there are many factors causing it. The factors from more important to less important are:
1) Previous exposure.
2) Motivation.
3) Attitude.
4) Learning style.
5) Aptitude
6) Personality.
However I think there are some advantages to having students of different levels because the students with higher level can help the other students: when lower level students see the accomplishments of higher level students they feel the chalenge to make an extra effort. Also higher students can help by modeling and encouraging them, and the students with less ability can be motivated for increase their performance. Another important reason is that this situation alows for the opportunity to learn about and accept differences.