To clarify things a little, real airspeed indicators work on air pressure. There is a pitot tube facing the front of the aircraft that records the pressure pushing against it. The faster the plane goes the more pressure there is and the higher the speed recorded. However the higher you go the less air pressure there is. This causes air speed indicators to read too low at alt, the higher you go the worse the problem is.
Like the real planes, the white needle on the AH speed indicator shows indicated airspeed and so reads low at alt. Indicated airspeed is more useful in flight anyway, since compression and flap deployment speeds are a function of this rather than true airspeed.
Because there used to be a lot of posts like your one on the BBS, HTC added a little red indicator to the dial showing true airspeed.