Generally, as a rule of thumb, the height of the dish determines the gain (signal strength) and the width of the dish determines its ability to collect and discern several different close orbital positions.
Thus, in the original Dish 500, the concept was probably "as long as we have to make it wider, why not also make it a little taller and help the rain fade situation".
Since then, new satellites were launched with a little stronger transponders, and so the thought with the Dish 1000 was "since we need to make it even wider, we should bring it back to the original 18 inches, in order to keep the overall size down". However, they did not originally have a good grasp of the poor reception of 129 in many areas, hence the new taller 100.2 dish.
The new 100.2 LNB (it doesn't have a separate model number?) also seems to be desirable for long term capability, since it feeds 3 dual tuner receivers and accepts 4 satellite locations (allows you to use another dish for 61.5 or 148).