The advantage of the K-3 III tech is that it makes the viewfinder brighter.
In the older cameras, the lights were illuminated through a window in the front wall of the prism. While it allowed the points to be projected through, it also allowed light from the lens to be lost through it. The new system does not need this, so more light is retained and the prism can be both bigger and brighter.
So while this doesn't directly answer why the points are illuminated differently, it explains why the new way of doing it is advantageous overall.
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