Bill Daniels wrote:
Like you, I'm only guesssing but it might be the DG system was intended to
be a stall warner rather than an AOA indicator although I grant the
difference is small. Ports on top of the wing might sense the progression
of flow separation at the root as a stall begins.

It does provide a stall warning function (not that the glider
particularly needs one), but it was also intended to be used to indicate
AoA for purposes of optimizing the flap settings. The calibration
procedure is clearly aimed at establishing both the high and low ends of
the optimal AoA range.

Marc