Visits: The number of sessions within a given time period. A visit is defined as a sequence of consecutive page views without a 30-minute break, or continuous activity for 12 hours. They most commonly consist of multiple image requests, however visitors that bounce can consist of a single page view.
Instances: The number of times a variable was defined. Each time Adobe Analytics sees a value within a variable, instances increment by one in that respective report.
Instances reporting higher than visits: A variable can be defined multiple times within a visit. For example, a user arrives at your site with Referrer A, then less than 30 minutes later arrives at your site again from Referrer B. In this case, two referrer instances occur within one visit.
Visits reporting higher than instances: Instances for individual line items are almost always greater than visits. The only time a visit can be counted without an instance is in context of conversion reports where a value persisted beyond the visit. That is, there is one instance, but the persisted value was seen in two visits. Total instances can be lower than total visits, as not all visits fire that specific variable in a visit. For example, total Tracking Code instances can be lower than visits, as not everyone comes to your site by clicking a campaign link.