1. What causes mixed-ability:
aptitude
previous exposure
attitude
motivation
learning style
personality
3. "Every group is a mixed-ability group." Indeed, every group is a mixed-ability group. There are always different learning rhythms and styles. Every subject is unique to learn a language and in life.
4. Mixed ability is usually considered to be a problem. Personally, I also think it is a problem. One of the advantages of having a mixed ability group is that you get different ideas and points of view. In addition, slow learners learn from advanced ones. But most of the time some get bored or others get stressed.