Why are true and indicated airspeed different?
Air pressure reduces with altitude, so while an individual molecule is striking the the aircraft at 100kt, there are fewer of them. This means there is less pressure placed on whatever surface is being struck.
The difference in indicated speed is simply one of a physical limitation with the air speed indicator - air speed indicators rely on dynamic pressure/ram air to give a readout. Because of that reduction in pressure, the readout begins to diverge from the true, physical speed as air pressure differs from the reference pressure of the gauge.
It would be possible to create an airspeed indicator which adjusts for this, in the same way as the altitude indicator does. However, as it happens, the planes flight characteristics also changes as pressure reduces in much the same way as the air speed indicator does. Because of this, for pilots, the IAS is the important speed outside of navigational purposes.
Hence, all design speeds are written in Indicated Air Speed and can be referred to at any altitude.