Why is there a difference between order volume in the Opportunity vs. Analytics?

Opportunities only track links that "count" towards the Opportunity (created within the time frame and generate 10 or more clicks). If you filter for an Opportunity in the Analytics tab it will show all data for those creators after the start date of the Opportunity, inclusive of data for sales that would not count towards the Opportunity (e.g. sales from a link that was generated before the start of the Opportunity).