A question similar to this was asked a few months ago. The answer is probably that you didn't notice that the bottle hadn't adopted it's maximum volume before you relased the cap. An alterative is that the contents of the bottle were under pressure and when you reduced the pressure by undoing the cap small bubbles were formed which increased the apparent volume of the liquid. Being very small these bubbles took some time to rise to the surface and escape. You can test this hypothesis on your next bottle by marking the original level, releasing the cap, marking the new level then leaving it for a few minutes to see if the level goes down again. Do this 10 minutes before you need to drink the contents.:o)