Here is the caused:
- One Lead can be added to multiple Campaigns
- But only the last Campaign associated to Lead prior to lead conversion will be bring into Primary Campaign Source in Opportunity
- When user convert Lead, Primary Campaign Source in Opportunity will be populated with last Campaign associated to Lead prior to the lead conversion
Report of Opportunity type
Last questions surface, so how do we know which is latest Campaign associated with a Lead or a Contact?
Easy... look at Campaign History related list in Lead or Contact page layout (make sure Campaign History related has been added) and field name Member First Associated.