Health goes down after returning an item

so, if you go back from lane with half your health and you buy and item, in this time your health goes full up, but if you realized that you bought the wrong item and return it, your health also goes back to when you bought the item. this can be a big game changer because, lets say there is a karthus, or anyone with a global, you go back karthus ult your return an item, karthus gets an unfair kill. or a more normal one, you wanna get to lane as quick as possible, you gotta wait that your health goes up. Again. if you wanna try it for your self is this easy. 1- go to lane 2- get damaged but not killed and go back to base 3- buy an item as soon as you get back to base and look at your health 4- let your health go back to max and click return now you'll see that your health has gone back to when ever you bought the item.