In SiteSpect, a visitor is counted when that visitor is exposed to a change in a Campaign. That visitor may also be counted when the visitor is exposed to the location in your website or app where the change exists, even if the visitor doesn't see the change because he or she is in the Control Group. In other words, that visitor is at the point where the change exists for other Variation Groups, so the visitor should be counted.
SiteSpect data is based on counting; as such, it is important for you to track this data. Using the Engine API for optimization requires tracking when a visitor is counted (for control and non-control experiences) in order to have accurate report data. Make sure to pass that counted state to SiteSpect in the API POST call and for each associated Variation Group.