I'm with Smajdalf. What did you expect to happen? What you got is exactly what I expected.
Please...
An LED can be very roughly approximated as a constant voltage load. This is why it is always said that you need some form of current limited for an LED, usually just a resistor for small indication LEDs. Directly attaching a constant voltage power supply (such as an output pin or large battery) to a constant voltage load is very dangerous. The mismatch in voltage will cause the device to either not operate at all (if the supply is too low) or draw excessive amounts of current (if the supply is too high).
What you're doing with the potentiometer and LED is that the LED is loading the potentiometer. The voltage divider formula is only valid is nothing is connected to the center of the resistors. As soon as you draw any current from the divider output (and an LED can use substantial current), the output voltage will drop. You need to use some extra math to calculate the actual output then (usually involving calculating the Thevenin equivalent circuit for the divider).