No error is shown, but the search for all emails doesn't match what is shown in the Case summary.
There is a difference in how the Case Information Summary total # of e-mails is retrieved compared to that of a search. (NOTE: Using a search criteria of Date Sent (AND) that is Before a date in the future will find all email for the custodians.)
The # of e-mails shown in summary is the total number of emails identified for all custodians in the Case. For example, if a single email is sent to 3 users it will show 3 emails in the summary.
When performing a search for all emails for the custodians only the unique email is shown. The difference from the previous example is that only 1 email is identified as only one index entry is created from the multiple custodian entries for the same email.
In this way the total amount of email exported from a Case will be accurate, as it will only export one copy of each email.
There is no issue with the difference in the results from the Case Information Summary and the Search as both are correct. They are simply enumerated differently.