Since no one has posted, here is a thought:
The ATI installers are set to optimize the drivers in relation to the hardware (video card) automatically, and may see a limitation in the older card - which results in the disabling of the alpha-blending features that would be enabled on a newer card.
An older driver would be designed for the current (at that time) technology, and therefore - if the optimization theory above is correct - the installer would not necessarily see a limitation in the older card, and would enable the alpha-blending features.
Again, this is just supposition based on a general understanding - and not a professional (or researched) basis of knowledge. 
