pylon:
The power used to light up the LEDs may slightly vary the voltage of the Arduino and that voltage is taken as a reference to measure the analog signal. As your signal varies in a small range this might explain the special behavior. Remove the LED switching part from the sketch and measure again. I'd guess that the behavior won't show up again.
Could i also try powering the sensor from an external source in your opinion? Or is it something that has nothing to do with what you wrote?