If your past issue URLs are http://[yoursite]/issue/[issue-name] then you are bypassing IssueM and using WordPress to display the article list. Chances are that it’s just using the standard archive template.
To ensure that the past issues use the same layout as your current issue, jump into your WordPress Admin interface, go to Articles > IssueM Settings and look for Links (Use Taxonomical links instead of shortcode based links for Issues). It’s likely not checked – if you check it, click Save Settings and then go back to your Past Issues page, you should find that the links end with /?issue=… and when you click through the format is the same as the current issue.
If you’d prefer to keep the links as /issue/[issue-name] then you’ll need to add a custom template to your theme called taxonomy-issuem_issue.php which will then be used by WordPress when responding to such requests.