in my opinion the most important factor in causing mixed abilities is the student's previous exposure to language, the rest of the factors would follow in this order:2.motivation 3.personality 4.learning style 5 attitude 6 aptitude.
I think most of the classes contain a mixed ability group , especially in adults classes, very different people coming with different personalities and backgrounds. but isn't this one on the teacher's duties and challenges at the same time? to adjust the teaching methods and materials and be able to respond to different learning needs.