Why does using a mana potion immediately restore 50 mana?

And just to get this out of the way: No, I'm not using time warp tonic. I checked that multiple times to make sure I wasn't just crazy, but mana pots immediately restore 50 mana when used even without the mastery. The question is: Why? Nothing about the item says it's supposed to. Did Riot change it to work this way and forget to tell anyone or change the tooltip?