I decided to look at the frequency distribution of values you get by using analogRead() on a floating (unconnected) analog pin.
With an Uno? Did you also test the Mega? What about the bigger Mega? The Teensys? What about when the Teensy is plugged into a breadboard? What if a neighbor pin is generating a PWM signal? What if a neighbor pin is HIGH? Or LOW? What if the board is powered by USB? What if the board is powered from VIN? What happens when the board is placed close to a desk lamp? Did you try the board near an OttLite? What happens when your hand is close to the pin? What about your mobile phone? Did you test after running the air conditioner a bit so the air is drier?
The point is, an unconnected pin is a terrible way to generate a random number. Even if you get good results some of the time there is no way to guarantee good results all of the time. AWOL's suggestion helps in the case that the voltage is changing but it obviously won't help in the copious cases that the voltage is not changing or is heavily influenced by something near.