With Simple Delivery, many sellers are getting an incorrect weight/size postage label, either by not putting in the correct weight in their listing, or finding that SD has come up with its own 'estimate'. Some sellers are using the inaccurate labels, because eBay says it's ok if SD chose the weight, but not if the seller got it wrong...
It looks like your refund is being handled by Royal Mail - so eBay must have arranged for RM to do the refunding, even though it is eBay that takes the postage cost from your buyer, to purchase the label for you...
I'm not sure I even understand what I've written... hopefully someone who's been successfully refunded will be able to shed some light on this... 😕