Look at it this way.
Ideal day, zero wind. IAS=GS=100kts.
Imagine a SUDDEN wind, on the nose at 20 kts. Effectivly now the air pressure on the pitot is due to a 120 knot INDICATED AS, due to the aircraft not being able to change its speed instantly. We therefore have GS100kts, IAS 120Knts.
Now Imagine the same wind appeared instantly on the tail. The air pressure on the pitot would now be due to an equivelent of 80 Kts, but the ground speed would still be 100 knts. Once the aircraft has accelerated back to an INDICATED 100 Kts the GS would be 120.
Its just a case of rembering how the ASI works.
(At least, that's my interpretation, for what it's worth)
