Shytorque - I think your QHI was trying to make an old CFS point about maintaining airspeed when manoeuvering close to the ground and turning downwind - the natural tendency at low level is to use groundspeed as your visual cue and consequently lose IAS as you turn downwind.
Jellycopter - there was another thread last year that talked about this phenomenon and I fully concur that it does happen (there, 3 heretics). Lots of clever sounding people banged on about inertial reference frames and much greek flute music was expounded. However, I think you have got it right, I'm just not clever enough to prove why.