Understanding Data from Email Reports

Understanding Data from Email Reports

Hello!

I'm trying to understand how the data featured in the Email Analytics report is generated.  Below is data from two CRM users:
MonthSentRespondedRepliedReceived
User 1January 20237511
February 202328421
March 202371681
April 202358050
May 2023950120
June 2023540210
TOTAL31315493
User 2January 2023521181
February 20231821134
March 20233637316
April 20231143327
May 20234313554
June 2023622320
TOTAL11912515292


If "Emails Replied to" is a subset of "Emails Sent" how is it possible that User 1 has sent less total emails than emails replied?  Also, why is User 2 showing nearly 0 Emails Received and few Emails Responded to?  User 2 is our biggest user of the CRM so we believe her emails received should be in the hundreds by now.

Both are members of our sales team.  User 1 has their sharing sent to Custom and User 2 is now Public, but was sharing under Custom for most of the time and was even Private for a brief time period.

We really want to use these email reports to monitor activity but are nervous to use them until we understand the numbers we are seeing.

Thanks!