I also didn't tell the boys that I took quite a bit more than we really needed for that batch of Polyjuice. I intended that batch as a "trial run," to see how well it worked. Having the ingredients for several batches of Polyjuice Potion at hand has come in useful, and having nobody but me know that I have it makes it even handier. You never know when I might want to be someone else, either to throw suspicion on them or off me.