Hi Leigh,
I suspect the (subtle) difference is that Issues are designed to be entered "by someone (Entered By), on behalf of someone else (Primary Contact)", so when the time to approve the Issue comes around, it is the Manager of the latter person (as opposed to the Manager of the person who Entered the Issue, in case they are different) who's in a position to give their Approval.
Conversely, the nature of Tasks is that they are Assigned to someone (Primary Assignee; noting that there could also be other assignees), so when the time comes to approve the Task, the approval workflow then goes to the Manager of that Primary Assignee.
Regards,
Doug