That would indicate not all who have seen the conversion page have entered the activity. Surely the conversion page existed before the activity was live and there are some visitors you are pulling into your custom Analytics reporting that have seen that conversion prior to the activity existing. If you want to only look at the data for the activity in quesiton plese user the special report created just for that. It can be accessed from: "Analytics > Reports > Adobe Target > Analytics for Target > Target Activities > Click on the Activity in question"
If this satisfies your query please like, mark as helpful and mark as answer. Otherwise lets keep the conversation going. Hope you have a wonderful day.
Are you saying that visitors viewed the conversion page first and then qualified for the activity, so this report is showing such visitors? This should not be possible based on how evars work (How eVars work in Adobe Analytics ). A4T variables act like an evar. So, the report should show whatever metrics/success events happened after the visitor qualified for the activity. Please correct me if my understanding is wrong.