Work Opaqueness: The more members in a Circle, the less accurate your team member's assessment of each other will be. Members tend to be the MOST familiar with the teammates' work they interact with the most often.
Various Areas of Expertise: Members do not always have experience with the effort required of all disciplines within the organization. For example, A developer may not know the rigor required of a marketer managing an active campaign. Due to this lack of experience, they may tend to under allocate to members of the marketing team.