The date you see when selecting messages for this report has nothing to do with the scheduled delivery. The date reflects the last date the message was modified, which is why the messages on the calendar are a lot more numerous than expected.

Note that if you have recently done a batch of archives on old email campaigns, they will have a more recent modify date and thus show up unexpectedly in reports. A useful trick to consider is to rename the archived messages by adding something like -ARCHIVED to the end, that way even if the modified date pushes them to the top of the list it will still be apparent that they're older.