This happens because discounts and memberships have no expiration period in the online sales shopping cart. When the customer logs in to purchase an item and then logs off without completing the order any memberships or discounts that were added to that order will still be in their cart the next time they attempt to purchase something online. 

You can determine if this is the case by creating the following query. This will allow you to see when they added the order to the cart, what discounts/promotions they used, and what membership they purchased: 
  1. Select the Analysis tab > Click Information Library
  2. Click Add an ad-hoc query > Choose the Sales Order source view
  3. From the middle pane, drag Lookup ID into Include Records where. Set this equal to the lookup ID of the order in question (8-sales order number). 
  4. From the middle pane, drag date added and date changed into Results fields to display. The date added will be the date they added the item to the cart originally, the date changed would be the day they completed the order and the day you got the email alert. 
  5. Expand Sales Order Item and highlight Sales Order Item Discount. From the middle drag Discount name into Results fields to display. 
  6. Expand Sales Order item Membership > Expand Sales Order item Membership Promotion Item > Highlight Membership Promotion. From the middle drag Name into Results fields to display. 
  7. (Optional) You could also Highlight the Sales Order Item Membership / Membership node and then drag membership program and membership level to see what membership they attempted to purchase.