D P
It sounds to me as if the cloud at 2.5A might go some way to explaining the pattern you observed. Since icing only occurs when there is visible moisture (in this case, cloud) the engine anti-ice is only used as we enter the cloud. In the case of our 757/767s, the temperature also has to be 10degC or below.
If the cloud was overcast at 6A (for example) then we would run downwind with the anti-ice on. As soon as we drop out of the cloud it would be switched off. As the normal 360/180 or 180/180 heading and speed combo are issued below 6A, the slowing up would all appear normal (anti-ice off). We then slide down the ILS nice and stable at 180kts, hit the top of the BKN layer, anti-ice back on again and the risk of a slight speed increase.
I think most of us would accept this increase of a few knots for a few seconds, since chances are we will be dropping the gear at 2A anyway and want to avoid the use of the speedbrake. If the speed does not drop back as we come out of the cloud and switch the anti-ice off, then I think most people would opt for a slightly earlier gear selection.
The pretty stiff breeze all the way to the ground, although eliminating the backing/veering headwind/tailwind problem, implies to me a degree of gustiness which would cause a fluctuating ASI reading. In such conditions it is sometimes hard to obtain the next flap setting because of speed limitation problems. Again, people may accept a temporary rise in speed (provided the magnitude is small) in the knowlege that as they will quickly pass through the cloud the previous steady state will be reaquired.
Turning to your question about IAS changing on entering cloud. In my experience it does, but I am not a meteorological expert so I cannot explain it with any degree of authority and the following should be taken with a large pinch of NaCl. As an aeroplane enters cloud the temperature changes quickly (albido effect?) as does the air density (moisture content). If the cloud is convective, then the vertical component of the wind will also changes. Whether all these effects are enough to influence the IAS, I couldn't tell you with any certainty or provide you with a reference text, but as a theory I think that they are a good starting point for further thought/discussion/research.
All the best
G W-H