This blog post is a contribution from Kevin Jacob Kurian, an engineer with the SharePoint Developer Support team.
When we edit a single instance of a recurring event, it actually creates a new instance of the event and deletes the old one. The old instance is deleted by setting certain attributes of that particular instance. This is per design.
Here’s an example of deleting a single instance of a recurring event:
SPListItem deleting_item = list.Items.Add();
deleting_item["MasterSeriesItemID"] = olditem.ID;
deleting_item["UID"] = olditem[“UID”];
// EventType is the attribute that defines that this event is to be deleted
deleting_item["EventType"] = 3;
// fRecurrence identifies that the event is a recurring event