The Activity Report and the Payments Report would only be exactly the same if all payments were received on the same day the invoices are generated.
If Payments are not made on the day invoices are generated, the reports will have different figures - it doesn't mean anything.
The Activity Report figures will be higher if many invoices have been generated and little payment has been received in that period of time.
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If it is the case, it could be relevant to look into the Ageing Debts report to find out who owes money to the practice and chase them up...
The Payments figures will be higher if many payments have been received and few invoices have been generated in a period of time.