The Dish Network User's Resource
When Dish introduced the 20 inch Dish 500, which allows reception from two satellites at different orbital positions, the larger size also led to increased signal strengths. Since in some parts of the country, signal strengths were near 100 points (it was never really a percentage), Dish introduced the 125 point signal strength scale. Now that some transponders at 119 are operating at double power, it became particularly important.
If the signal strength scale had not been expanded it could be very difficult to find the peak signal strength with a scale that only went up to 100.