Opened is higher than delivered

Marketo retains the email send and delivered activities for a period of 90 days, whereas the email open and click activities are retained for a period of 25 months from the date of the activity (refer all the details of the data retention policy here). The smart list may be showing the open email activities for which the corresponding email delivered activities may have got purged after the period of 90 days from the activity logs. It's also worth mentioning here that Marketo logs only a single open activity for a email + lead combo in the activity logs, which is not the case for the email delivered activity. Aggregated data in the email performance reports in the analytics section can still show the send and delivered data which is older than 90 days (unless you add the filters in the report SL), and should allow you to be able to lookback sent/delivered data along with the open/clicked email data for up to a period of 25 months from the date of activity.