Product Quantity is updated by system.
In the RMS product edit page, there is a inventory setting under "Quantity", called "Return Products to Inventory on Canceled Orders".
When merchants enable the setting, the quantity or this product will increase when order has been cancelled.
If merchants don't want this happen, they could uncheck this setting.
This setting is only available for limited number products, if you choose "Unlimited" in inventory, the "Return Products to Inventory on Canceled Orders" is not available.