I'd imagine it might be because the <t> and <tt> are targeting two different things, and if one is able to be hit with Mercy/Misery and the other isn't it might just interrupt the 2nd check later on in the macro or just confuse the game or something. Or maybe the HP check only deals with what you are actively targeting. It's different than the other skills which just have to check if the cooldown is ticking or not.