But are you always starting and stopping at the same altitude? If not, then is the rate of climb the same at different altitudes? That would make a difference.I considered the first (as the recording device is inside the cockpit measuring the pressure there) but don't think that's it, as I'm only looking at the change in altitude; the actual altitude doesn't matter. Whatever variation or error there is should be relatively constant during each climb segment.
Also, at different AoA you could be getting some errors in your air speed caused by the pitot tube not being in line with the air flow.