I've just encountered a bug on your live demo site:
Using a mobile device (or spoofing the user agent), go to a product page (e.g http://demo.smartsto...n/apple-watch-2) and change the quantity to anything other than 1 and add it to the cart. Only one of the selected product is added to the shopping cart (regardless of the quantity specified). Updating the quantity from the shopping cart page works fine.
Where is the code responsible for this?