If you pump tranny fluid IN from the top, it will fall to the bottom of the reservior and go directly back into the tranny, with no time spent in the radiator to cool it off.
By pumping it into the reservior from the bottom, it is required to travel up the complete length of the chamber, cool itself off, and then flow back into the tranny.
Plus the tranny fluid operates at a higher temp than radiator cooler, so the difference between 210/180 from top to bottom of radiator really doesn't matter that much in cooling the tranny fluid.
