Ptkay, you're looking at the data from the flight data recorder, and my guess would be that the quantization of the barometric altimeter data is rather crude there. Also, it is a pressure altitude reading (look it up). As of the RA, its data are right there (see the red line which is annotated "Alt.r: Radio altitude"). And it might've been exactly what they were doing — flying by the RA (this hypothesis has been discussed here to death).