There are a number of factors. When looking at individual attorneys you may wish to look at their rankings on their page as opposed to their contributor level. The ranking takes into consideration experience, ratings by peers, contributions to the legal community and disciplinary history. This may be helpful.
Contributor level just means that the attorney has answered more questions. PRO means that the attorney pays AVVO for an enhanced service.
The attorney rating is based on factors such as experience, legal ability, reputation in the community, ethics, etc. look for the attorney rating.