I think I can help you out there - it's the same reason for not including the really fantastic fish: even though it happens in real life, you wouldn't believe it in this instance.
Because you can't actually *see* the glass, can't appreciate that it is there, a starfish at the top of the glass would just look like a starfish that was hanging about in water looking sheepish. We would wonder why it doesn't sink back to the sea bed.
When the starfish is lower down, somehow we still believe in the virtual existence of the glass and it looks good.
At least, that's what *I* think!
