The Pensieve does not float in the book.
Harry glanced hopelessly at Dumbledore’s deserted frame, which hung directly behind the headmaster’s chair, then turned his back on it. The stone Pensieve lay in the cabinet where it had always been. Harry heaved it onto the desk and poured Snape’s memories into the wide basin with its runic markings around the edge. To escape into someone else’s head would be a blessed relief...Nothing that even Snape had left him could be worse than his own thoughts. The memories swirled, silver white and strange, and without hesitating, with a feeling of reckless abandonment, as though this would assuage his torturing grief, Harry dived.
Harry Potter and the Deathly Hallows, Chapter 33, The Prince’s Tale
I think it was added to make the moment look special because it was revealing of real Severus Snape.