Data may take up to four hours to populate in Inbox Monitor for existing ESP Connect integrations, so you may see differences in the data until all data has been displayed.
Salesforce Marketing Cloud may also decrease send volume totals after bounced messages are accounted for in their calculation, which may not occur until the next day. The ESP Connect integration cannot adjust the send volume total in Return Path's user interface when it decreases over time, so you may see data differences between Return Path and Salesforce Marketing Cloud. Return Path's user interface does adjust for increases in the data, within a 5 day window. If the Delivered in Return Path UI is higher than Salesforce Marketing Cloud, subtracting the Bounces from the Sent in ESP Connect data should provide a closer data match to Salesforce Marketing Cloud Total Delivered. Future improvements to ESP Connect will account for decreases in send volume so the values match between Return Path and Salesforce Marketing Cloud.