Inventory/upgrade pricing is dynamic. The obvious test here would have been to go back and see what price was showing on the desktop and see if it was still the same. But since you already bought the upgrade, it's too late for that. There's some speculation that there is fare price discrimination based on your identity which is why some people suggest checking fare pricing anonymously by clearing cookies, using anonymous browser mode, using a VPN, etc.. But since they already know your identity when you are checking upgrade pricing on an existing itinerary, that would be pointless to do in this case. What would be the point of price discrimination for the same person simply based on device type or form of network access?