The reason that you're seeing a discrepancy could be due to a few reasons:
1) The Subscriber Retention report only includes the 1st subscription in an account. If a subscriber has an active subscription that they created in, let's say June 2019, the new subscription that converted in August 2019 would not be included since we only show one per account.
2) Past due invoices are not included. If a subscription has a past due, unpaid invoice on the account, it will not count toward the Subscriber Retention report. The >$0 invoice must be paid to be included in that figure.
3) No $0 invoices are included. Besides trials, we also don't include invoices that were completely discounted to $0 due to a coupon, credit, or gift card application.
If you want to view your list of active, paying subscribers, your best bet is to use the Subscriptions export (Status: Live, Time Range: All time) and to use the trial_starts_at and trial_ends_at columns to determine which subscriptions are still in trial and which have converted to paying subscribers.