Why is the sum of Paying Subscribers in the Subscriber Retention report lower than the total of Subscriptions per Plan in the Plans dashboard?

These two dashboards have different requirements as far as which types of subscriptions are included in their calculations, so that's why the totals you're seeing do not match up. 

For example , the Subscriber Retention dashboard does NOT include subscriptions in trial or second subscriptions on an account (If an existing subscriber purchases a new subscription, bringing the account's total live subscription count to '2,'  the 2nd subscription will not impact or count toward this report). Therefore, if an account has 2 live subscriptions on their account, only the first one that was added (and is currently paying) would be counted as a paying subscription to the Subscriber Retention report. 

On the other hand, the Subscriptions by Plans dashboard DOES include trial subscriptions and all subscriptions that are active per account. This means that if one account has 3 live subscriptions, ALL of them would count toward their respective plan in the Subscriptions by Plans dashboard. 

This is why you're seeing a higher total for the Subscriptions by Plans total versus the Subscriber Retention paying subscribers report.