Why are the positions of an aircraft's angle-of-attack vanes dependent on airspeed?
It's because the vane isn't weathervaning into the freestream, it's weathervaning into the local flow a few inches away from the skin, and because of the curvature of the nose, this flow changes with body AOA (and therefore speed) with some variation relative to the change in freestream AOA.  The Stall Protection Computer software will have calibration tables derived from development flight testing to compensate for most of this.