Hi,
When you delete an order, the delete function just delete it.
For the moment, the best is to remove all products from the order and then delete the order.
When you remove a product from an order, the quantity is updated.
I have included this request in our TODO list.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.