Can you provide more information about your email program and status setup? Since statuses must be set up in Smart Campaigns, it's tough to say without seeing what you have going on.
Send Email is logged by Marketo when the campaign hits that flow step for the lead. If you a running a campaign with large #s of leads, the actual time the email goes out into the internet can be tracked more closely by the "delivered email" event. Especially if there is a lot of email scripting, tokens, dynamic content, etc. you'll see time discrepancy.